597 lines
29 KiB
ReStructuredText
597 lines
29 KiB
ReStructuredText
.. _hugetlbfs_reserve:
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=====================
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Hugetlbfs Reservation
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=====================
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Overview
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========
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Huge pages as described at :ref:`hugetlbpage` are typically
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preallocated for application use. These huge pages are instantiated in a
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task's address space at page fault time if the VMA indicates huge pages are
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to be used. If no huge page exists at page fault time, the task is sent
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a SIGBUS and often dies an unhappy death. Shortly after huge page support
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was added, it was determined that it would be better to detect a shortage
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of huge pages at mmap() time. The idea is that if there were not enough
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huge pages to cover the mapping, the mmap() would fail. This was first
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done with a simple check in the code at mmap() time to determine if there
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were enough free huge pages to cover the mapping. Like most things in the
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kernel, the code has evolved over time. However, the basic idea was to
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'reserve' huge pages at mmap() time to ensure that huge pages would be
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available for page faults in that mapping. The description below attempts to
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describe how huge page reserve processing is done in the v4.10 kernel.
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Audience
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========
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This description is primarily targeted at kernel developers who are modifying
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hugetlbfs code.
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The Data Structures
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===================
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resv_huge_pages
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This is a global (per-hstate) count of reserved huge pages. Reserved
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huge pages are only available to the task which reserved them.
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Therefore, the number of huge pages generally available is computed
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as (``free_huge_pages - resv_huge_pages``).
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Reserve Map
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A reserve map is described by the structure::
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struct resv_map {
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struct kref refs;
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spinlock_t lock;
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struct list_head regions;
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long adds_in_progress;
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struct list_head region_cache;
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long region_cache_count;
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};
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There is one reserve map for each huge page mapping in the system.
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The regions list within the resv_map describes the regions within
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the mapping. A region is described as::
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struct file_region {
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struct list_head link;
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long from;
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long to;
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};
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The 'from' and 'to' fields of the file region structure are huge page
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indices into the mapping. Depending on the type of mapping, a
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region in the reserv_map may indicate reservations exist for the
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range, or reservations do not exist.
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Flags for MAP_PRIVATE Reservations
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These are stored in the bottom bits of the reservation map pointer.
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``#define HPAGE_RESV_OWNER (1UL << 0)``
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Indicates this task is the owner of the reservations
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associated with the mapping.
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``#define HPAGE_RESV_UNMAPPED (1UL << 1)``
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Indicates task originally mapping this range (and creating
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reserves) has unmapped a page from this task (the child)
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due to a failed COW.
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Page Flags
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The PagePrivate page flag is used to indicate that a huge page
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reservation must be restored when the huge page is freed. More
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details will be discussed in the "Freeing huge pages" section.
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Reservation Map Location (Private or Shared)
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============================================
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A huge page mapping or segment is either private or shared. If private,
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it is typically only available to a single address space (task). If shared,
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it can be mapped into multiple address spaces (tasks). The location and
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semantics of the reservation map is significantly different for the two types
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of mappings. Location differences are:
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- For private mappings, the reservation map hangs off the VMA structure.
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Specifically, vma->vm_private_data. This reserve map is created at the
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time the mapping (mmap(MAP_PRIVATE)) is created.
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- For shared mappings, the reservation map hangs off the inode. Specifically,
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inode->i_mapping->private_data. Since shared mappings are always backed
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by files in the hugetlbfs filesystem, the hugetlbfs code ensures each inode
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contains a reservation map. As a result, the reservation map is allocated
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when the inode is created.
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Creating Reservations
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=====================
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Reservations are created when a huge page backed shared memory segment is
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created (shmget(SHM_HUGETLB)) or a mapping is created via mmap(MAP_HUGETLB).
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These operations result in a call to the routine hugetlb_reserve_pages()::
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int hugetlb_reserve_pages(struct inode *inode,
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long from, long to,
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struct vm_area_struct *vma,
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vm_flags_t vm_flags)
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The first thing hugetlb_reserve_pages() does is check if the NORESERVE
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flag was specified in either the shmget() or mmap() call. If NORESERVE
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was specified, then this routine returns immediately as no reservations
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are desired.
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The arguments 'from' and 'to' are huge page indices into the mapping or
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underlying file. For shmget(), 'from' is always 0 and 'to' corresponds to
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the length of the segment/mapping. For mmap(), the offset argument could
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be used to specify the offset into the underlying file. In such a case,
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the 'from' and 'to' arguments have been adjusted by this offset.
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One of the big differences between PRIVATE and SHARED mappings is the way
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in which reservations are represented in the reservation map.
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- For shared mappings, an entry in the reservation map indicates a reservation
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exists or did exist for the corresponding page. As reservations are
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consumed, the reservation map is not modified.
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- For private mappings, the lack of an entry in the reservation map indicates
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a reservation exists for the corresponding page. As reservations are
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consumed, entries are added to the reservation map. Therefore, the
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reservation map can also be used to determine which reservations have
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been consumed.
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For private mappings, hugetlb_reserve_pages() creates the reservation map and
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hangs it off the VMA structure. In addition, the HPAGE_RESV_OWNER flag is set
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to indicate this VMA owns the reservations.
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The reservation map is consulted to determine how many huge page reservations
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are needed for the current mapping/segment. For private mappings, this is
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always the value (to - from). However, for shared mappings it is possible that
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some reservations may already exist within the range (to - from). See the
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section :ref:`Reservation Map Modifications <resv_map_modifications>`
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for details on how this is accomplished.
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The mapping may be associated with a subpool. If so, the subpool is consulted
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to ensure there is sufficient space for the mapping. It is possible that the
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subpool has set aside reservations that can be used for the mapping. See the
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section :ref:`Subpool Reservations <sub_pool_resv>` for more details.
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After consulting the reservation map and subpool, the number of needed new
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reservations is known. The routine hugetlb_acct_memory() is called to check
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for and take the requested number of reservations. hugetlb_acct_memory()
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calls into routines that potentially allocate and adjust surplus page counts.
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However, within those routines the code is simply checking to ensure there
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are enough free huge pages to accommodate the reservation. If there are,
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the global reservation count resv_huge_pages is adjusted something like the
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following::
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if (resv_needed <= (resv_huge_pages - free_huge_pages))
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resv_huge_pages += resv_needed;
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Note that the global lock hugetlb_lock is held when checking and adjusting
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these counters.
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If there were enough free huge pages and the global count resv_huge_pages
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was adjusted, then the reservation map associated with the mapping is
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modified to reflect the reservations. In the case of a shared mapping, a
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file_region will exist that includes the range 'from' - 'to'. For private
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mappings, no modifications are made to the reservation map as lack of an
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entry indicates a reservation exists.
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If hugetlb_reserve_pages() was successful, the global reservation count and
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reservation map associated with the mapping will be modified as required to
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ensure reservations exist for the range 'from' - 'to'.
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.. _consume_resv:
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Consuming Reservations/Allocating a Huge Page
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=============================================
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Reservations are consumed when huge pages associated with the reservations
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are allocated and instantiated in the corresponding mapping. The allocation
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is performed within the routine alloc_huge_page()::
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struct page *alloc_huge_page(struct vm_area_struct *vma,
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unsigned long addr, int avoid_reserve)
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alloc_huge_page is passed a VMA pointer and a virtual address, so it can
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consult the reservation map to determine if a reservation exists. In addition,
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alloc_huge_page takes the argument avoid_reserve which indicates reserves
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should not be used even if it appears they have been set aside for the
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specified address. The avoid_reserve argument is most often used in the case
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of Copy on Write and Page Migration where additional copies of an existing
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page are being allocated.
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The helper routine vma_needs_reservation() is called to determine if a
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reservation exists for the address within the mapping(vma). See the section
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:ref:`Reservation Map Helper Routines <resv_map_helpers>` for detailed
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information on what this routine does.
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The value returned from vma_needs_reservation() is generally
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0 or 1. 0 if a reservation exists for the address, 1 if no reservation exists.
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If a reservation does not exist, and there is a subpool associated with the
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mapping the subpool is consulted to determine if it contains reservations.
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If the subpool contains reservations, one can be used for this allocation.
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However, in every case the avoid_reserve argument overrides the use of
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a reservation for the allocation. After determining whether a reservation
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exists and can be used for the allocation, the routine dequeue_huge_page_vma()
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is called. This routine takes two arguments related to reservations:
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- avoid_reserve, this is the same value/argument passed to alloc_huge_page()
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- chg, even though this argument is of type long only the values 0 or 1 are
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passed to dequeue_huge_page_vma. If the value is 0, it indicates a
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reservation exists (see the section "Memory Policy and Reservations" for
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possible issues). If the value is 1, it indicates a reservation does not
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exist and the page must be taken from the global free pool if possible.
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The free lists associated with the memory policy of the VMA are searched for
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a free page. If a page is found, the value free_huge_pages is decremented
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when the page is removed from the free list. If there was a reservation
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associated with the page, the following adjustments are made::
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SetPagePrivate(page); /* Indicates allocating this page consumed
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* a reservation, and if an error is
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* encountered such that the page must be
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* freed, the reservation will be restored. */
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resv_huge_pages--; /* Decrement the global reservation count */
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Note, if no huge page can be found that satisfies the VMA's memory policy
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an attempt will be made to allocate one using the buddy allocator. This
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brings up the issue of surplus huge pages and overcommit which is beyond
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the scope reservations. Even if a surplus page is allocated, the same
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reservation based adjustments as above will be made: SetPagePrivate(page) and
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resv_huge_pages--.
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After obtaining a new huge page, (page)->private is set to the value of
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the subpool associated with the page if it exists. This will be used for
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subpool accounting when the page is freed.
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The routine vma_commit_reservation() is then called to adjust the reserve
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map based on the consumption of the reservation. In general, this involves
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ensuring the page is represented within a file_region structure of the region
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map. For shared mappings where the reservation was present, an entry
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in the reserve map already existed so no change is made. However, if there
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was no reservation in a shared mapping or this was a private mapping a new
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entry must be created.
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It is possible that the reserve map could have been changed between the call
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to vma_needs_reservation() at the beginning of alloc_huge_page() and the
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call to vma_commit_reservation() after the page was allocated. This would
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be possible if hugetlb_reserve_pages was called for the same page in a shared
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mapping. In such cases, the reservation count and subpool free page count
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will be off by one. This rare condition can be identified by comparing the
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return value from vma_needs_reservation and vma_commit_reservation. If such
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a race is detected, the subpool and global reserve counts are adjusted to
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compensate. See the section
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:ref:`Reservation Map Helper Routines <resv_map_helpers>` for more
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information on these routines.
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Instantiate Huge Pages
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======================
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After huge page allocation, the page is typically added to the page tables
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of the allocating task. Before this, pages in a shared mapping are added
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to the page cache and pages in private mappings are added to an anonymous
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reverse mapping. In both cases, the PagePrivate flag is cleared. Therefore,
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when a huge page that has been instantiated is freed no adjustment is made
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to the global reservation count (resv_huge_pages).
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Freeing Huge Pages
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==================
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Huge page freeing is performed by the routine free_huge_page(). This routine
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is the destructor for hugetlbfs compound pages. As a result, it is only
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passed a pointer to the page struct. When a huge page is freed, reservation
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accounting may need to be performed. This would be the case if the page was
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associated with a subpool that contained reserves, or the page is being freed
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on an error path where a global reserve count must be restored.
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The page->private field points to any subpool associated with the page.
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If the PagePrivate flag is set, it indicates the global reserve count should
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be adjusted (see the section
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:ref:`Consuming Reservations/Allocating a Huge Page <consume_resv>`
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for information on how these are set).
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The routine first calls hugepage_subpool_put_pages() for the page. If this
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routine returns a value of 0 (which does not equal the value passed 1) it
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indicates reserves are associated with the subpool, and this newly free page
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must be used to keep the number of subpool reserves above the minimum size.
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Therefore, the global resv_huge_pages counter is incremented in this case.
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If the PagePrivate flag was set in the page, the global resv_huge_pages counter
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will always be incremented.
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.. _sub_pool_resv:
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Subpool Reservations
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====================
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There is a struct hstate associated with each huge page size. The hstate
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tracks all huge pages of the specified size. A subpool represents a subset
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of pages within a hstate that is associated with a mounted hugetlbfs
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filesystem.
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When a hugetlbfs filesystem is mounted a min_size option can be specified
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which indicates the minimum number of huge pages required by the filesystem.
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If this option is specified, the number of huge pages corresponding to
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min_size are reserved for use by the filesystem. This number is tracked in
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the min_hpages field of a struct hugepage_subpool. At mount time,
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hugetlb_acct_memory(min_hpages) is called to reserve the specified number of
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huge pages. If they can not be reserved, the mount fails.
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The routines hugepage_subpool_get/put_pages() are called when pages are
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obtained from or released back to a subpool. They perform all subpool
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accounting, and track any reservations associated with the subpool.
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hugepage_subpool_get/put_pages are passed the number of huge pages by which
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to adjust the subpool 'used page' count (down for get, up for put). Normally,
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they return the same value that was passed or an error if not enough pages
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exist in the subpool.
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However, if reserves are associated with the subpool a return value less
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than the passed value may be returned. This return value indicates the
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number of additional global pool adjustments which must be made. For example,
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suppose a subpool contains 3 reserved huge pages and someone asks for 5.
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The 3 reserved pages associated with the subpool can be used to satisfy part
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of the request. But, 2 pages must be obtained from the global pools. To
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relay this information to the caller, the value 2 is returned. The caller
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is then responsible for attempting to obtain the additional two pages from
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the global pools.
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COW and Reservations
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====================
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Since shared mappings all point to and use the same underlying pages, the
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biggest reservation concern for COW is private mappings. In this case,
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two tasks can be pointing at the same previously allocated page. One task
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attempts to write to the page, so a new page must be allocated so that each
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task points to its own page.
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When the page was originally allocated, the reservation for that page was
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consumed. When an attempt to allocate a new page is made as a result of
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COW, it is possible that no free huge pages are free and the allocation
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will fail.
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When the private mapping was originally created, the owner of the mapping
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was noted by setting the HPAGE_RESV_OWNER bit in the pointer to the reservation
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map of the owner. Since the owner created the mapping, the owner owns all
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the reservations associated with the mapping. Therefore, when a write fault
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occurs and there is no page available, different action is taken for the owner
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and non-owner of the reservation.
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In the case where the faulting task is not the owner, the fault will fail and
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the task will typically receive a SIGBUS.
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If the owner is the faulting task, we want it to succeed since it owned the
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original reservation. To accomplish this, the page is unmapped from the
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non-owning task. In this way, the only reference is from the owning task.
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In addition, the HPAGE_RESV_UNMAPPED bit is set in the reservation map pointer
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of the non-owning task. The non-owning task may receive a SIGBUS if it later
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faults on a non-present page. But, the original owner of the
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mapping/reservation will behave as expected.
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.. _resv_map_modifications:
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Reservation Map Modifications
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=============================
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The following low level routines are used to make modifications to a
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reservation map. Typically, these routines are not called directly. Rather,
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a reservation map helper routine is called which calls one of these low level
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routines. These low level routines are fairly well documented in the source
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code (mm/hugetlb.c). These routines are::
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long region_chg(struct resv_map *resv, long f, long t);
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long region_add(struct resv_map *resv, long f, long t);
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void region_abort(struct resv_map *resv, long f, long t);
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long region_count(struct resv_map *resv, long f, long t);
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Operations on the reservation map typically involve two operations:
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1) region_chg() is called to examine the reserve map and determine how
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many pages in the specified range [f, t) are NOT currently represented.
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The calling code performs global checks and allocations to determine if
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there are enough huge pages for the operation to succeed.
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2)
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a) If the operation can succeed, region_add() is called to actually modify
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the reservation map for the same range [f, t) previously passed to
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region_chg().
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b) If the operation can not succeed, region_abort is called for the same
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range [f, t) to abort the operation.
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Note that this is a two step process where region_add() and region_abort()
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are guaranteed to succeed after a prior call to region_chg() for the same
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range. region_chg() is responsible for pre-allocating any data structures
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necessary to ensure the subsequent operations (specifically region_add()))
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will succeed.
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As mentioned above, region_chg() determines the number of pages in the range
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which are NOT currently represented in the map. This number is returned to
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the caller. region_add() returns the number of pages in the range added to
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the map. In most cases, the return value of region_add() is the same as the
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return value of region_chg(). However, in the case of shared mappings it is
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possible for changes to the reservation map to be made between the calls to
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region_chg() and region_add(). In this case, the return value of region_add()
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will not match the return value of region_chg(). It is likely that in such
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cases global counts and subpool accounting will be incorrect and in need of
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adjustment. It is the responsibility of the caller to check for this condition
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and make the appropriate adjustments.
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The routine region_del() is called to remove regions from a reservation map.
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It is typically called in the following situations:
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- When a file in the hugetlbfs filesystem is being removed, the inode will
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be released and the reservation map freed. Before freeing the reservation
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map, all the individual file_region structures must be freed. In this case
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region_del is passed the range [0, LONG_MAX).
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- When a hugetlbfs file is being truncated. In this case, all allocated pages
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after the new file size must be freed. In addition, any file_region entries
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in the reservation map past the new end of file must be deleted. In this
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case, region_del is passed the range [new_end_of_file, LONG_MAX).
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- When a hole is being punched in a hugetlbfs file. In this case, huge pages
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are removed from the middle of the file one at a time. As the pages are
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removed, region_del() is called to remove the corresponding entry from the
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reservation map. In this case, region_del is passed the range
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[page_idx, page_idx + 1).
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In every case, region_del() will return the number of pages removed from the
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reservation map. In VERY rare cases, region_del() can fail. This can only
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happen in the hole punch case where it has to split an existing file_region
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entry and can not allocate a new structure. In this error case, region_del()
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will return -ENOMEM. The problem here is that the reservation map will
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indicate that there is a reservation for the page. However, the subpool and
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global reservation counts will not reflect the reservation. To handle this
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situation, the routine hugetlb_fix_reserve_counts() is called to adjust the
|
|
counters so that they correspond with the reservation map entry that could
|
|
not be deleted.
|
|
|
|
region_count() is called when unmapping a private huge page mapping. In
|
|
private mappings, the lack of a entry in the reservation map indicates that
|
|
a reservation exists. Therefore, by counting the number of entries in the
|
|
reservation map we know how many reservations were consumed and how many are
|
|
outstanding (outstanding = (end - start) - region_count(resv, start, end)).
|
|
Since the mapping is going away, the subpool and global reservation counts
|
|
are decremented by the number of outstanding reservations.
|
|
|
|
.. _resv_map_helpers:
|
|
|
|
Reservation Map Helper Routines
|
|
===============================
|
|
|
|
Several helper routines exist to query and modify the reservation maps.
|
|
These routines are only interested with reservations for a specific huge
|
|
page, so they just pass in an address instead of a range. In addition,
|
|
they pass in the associated VMA. From the VMA, the type of mapping (private
|
|
or shared) and the location of the reservation map (inode or VMA) can be
|
|
determined. These routines simply call the underlying routines described
|
|
in the section "Reservation Map Modifications". However, they do take into
|
|
account the 'opposite' meaning of reservation map entries for private and
|
|
shared mappings and hide this detail from the caller::
|
|
|
|
long vma_needs_reservation(struct hstate *h,
|
|
struct vm_area_struct *vma,
|
|
unsigned long addr)
|
|
|
|
This routine calls region_chg() for the specified page. If no reservation
|
|
exists, 1 is returned. If a reservation exists, 0 is returned::
|
|
|
|
long vma_commit_reservation(struct hstate *h,
|
|
struct vm_area_struct *vma,
|
|
unsigned long addr)
|
|
|
|
This calls region_add() for the specified page. As in the case of region_chg
|
|
and region_add, this routine is to be called after a previous call to
|
|
vma_needs_reservation. It will add a reservation entry for the page. It
|
|
returns 1 if the reservation was added and 0 if not. The return value should
|
|
be compared with the return value of the previous call to
|
|
vma_needs_reservation. An unexpected difference indicates the reservation
|
|
map was modified between calls::
|
|
|
|
void vma_end_reservation(struct hstate *h,
|
|
struct vm_area_struct *vma,
|
|
unsigned long addr)
|
|
|
|
This calls region_abort() for the specified page. As in the case of region_chg
|
|
and region_abort, this routine is to be called after a previous call to
|
|
vma_needs_reservation. It will abort/end the in progress reservation add
|
|
operation::
|
|
|
|
long vma_add_reservation(struct hstate *h,
|
|
struct vm_area_struct *vma,
|
|
unsigned long addr)
|
|
|
|
This is a special wrapper routine to help facilitate reservation cleanup
|
|
on error paths. It is only called from the routine restore_reserve_on_error().
|
|
This routine is used in conjunction with vma_needs_reservation in an attempt
|
|
to add a reservation to the reservation map. It takes into account the
|
|
different reservation map semantics for private and shared mappings. Hence,
|
|
region_add is called for shared mappings (as an entry present in the map
|
|
indicates a reservation), and region_del is called for private mappings (as
|
|
the absence of an entry in the map indicates a reservation). See the section
|
|
"Reservation cleanup in error paths" for more information on what needs to
|
|
be done on error paths.
|
|
|
|
|
|
Reservation Cleanup in Error Paths
|
|
==================================
|
|
|
|
As mentioned in the section
|
|
:ref:`Reservation Map Helper Routines <resv_map_helpers>`, reservation
|
|
map modifications are performed in two steps. First vma_needs_reservation
|
|
is called before a page is allocated. If the allocation is successful,
|
|
then vma_commit_reservation is called. If not, vma_end_reservation is called.
|
|
Global and subpool reservation counts are adjusted based on success or failure
|
|
of the operation and all is well.
|
|
|
|
Additionally, after a huge page is instantiated the PagePrivate flag is
|
|
cleared so that accounting when the page is ultimately freed is correct.
|
|
|
|
However, there are several instances where errors are encountered after a huge
|
|
page is allocated but before it is instantiated. In this case, the page
|
|
allocation has consumed the reservation and made the appropriate subpool,
|
|
reservation map and global count adjustments. If the page is freed at this
|
|
time (before instantiation and clearing of PagePrivate), then free_huge_page
|
|
will increment the global reservation count. However, the reservation map
|
|
indicates the reservation was consumed. This resulting inconsistent state
|
|
will cause the 'leak' of a reserved huge page. The global reserve count will
|
|
be higher than it should and prevent allocation of a pre-allocated page.
|
|
|
|
The routine restore_reserve_on_error() attempts to handle this situation. It
|
|
is fairly well documented. The intention of this routine is to restore
|
|
the reservation map to the way it was before the page allocation. In this
|
|
way, the state of the reservation map will correspond to the global reservation
|
|
count after the page is freed.
|
|
|
|
The routine restore_reserve_on_error itself may encounter errors while
|
|
attempting to restore the reservation map entry. In this case, it will
|
|
simply clear the PagePrivate flag of the page. In this way, the global
|
|
reserve count will not be incremented when the page is freed. However, the
|
|
reservation map will continue to look as though the reservation was consumed.
|
|
A page can still be allocated for the address, but it will not use a reserved
|
|
page as originally intended.
|
|
|
|
There is some code (most notably userfaultfd) which can not call
|
|
restore_reserve_on_error. In this case, it simply modifies the PagePrivate
|
|
so that a reservation will not be leaked when the huge page is freed.
|
|
|
|
|
|
Reservations and Memory Policy
|
|
==============================
|
|
Per-node huge page lists existed in struct hstate when git was first used
|
|
to manage Linux code. The concept of reservations was added some time later.
|
|
When reservations were added, no attempt was made to take memory policy
|
|
into account. While cpusets are not exactly the same as memory policy, this
|
|
comment in hugetlb_acct_memory sums up the interaction between reservations
|
|
and cpusets/memory policy::
|
|
|
|
/*
|
|
* When cpuset is configured, it breaks the strict hugetlb page
|
|
* reservation as the accounting is done on a global variable. Such
|
|
* reservation is completely rubbish in the presence of cpuset because
|
|
* the reservation is not checked against page availability for the
|
|
* current cpuset. Application can still potentially OOM'ed by kernel
|
|
* with lack of free htlb page in cpuset that the task is in.
|
|
* Attempt to enforce strict accounting with cpuset is almost
|
|
* impossible (or too ugly) because cpuset is too fluid that
|
|
* task or memory node can be dynamically moved between cpusets.
|
|
*
|
|
* The change of semantics for shared hugetlb mapping with cpuset is
|
|
* undesirable. However, in order to preserve some of the semantics,
|
|
* we fall back to check against current free page availability as
|
|
* a best attempt and hopefully to minimize the impact of changing
|
|
* semantics that cpuset has.
|
|
*/
|
|
|
|
Huge page reservations were added to prevent unexpected page allocation
|
|
failures (OOM) at page fault time. However, if an application makes use
|
|
of cpusets or memory policy there is no guarantee that huge pages will be
|
|
available on the required nodes. This is true even if there are a sufficient
|
|
number of global reservations.
|
|
|
|
Hugetlbfs regression testing
|
|
============================
|
|
|
|
The most complete set of hugetlb tests are in the libhugetlbfs repository.
|
|
If you modify any hugetlb related code, use the libhugetlbfs test suite
|
|
to check for regressions. In addition, if you add any new hugetlb
|
|
functionality, please add appropriate tests to libhugetlbfs.
|
|
|
|
--
|
|
Mike Kravetz, 7 April 2017
|