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[PATCH] Update some VFS documentation 

Flesh out the description of the address_space operations.

Signed-off-by: Neil Brown <neilb@suse.de>
Cc: Avishay Traeger <atraeger@cs.sunysb.edu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
NeilBrown 2006-03-25 03:07:56 -08:00 committed by Linus Torvalds
parent d5ee4ea833
commit 341546f5ad
1 changed files with 190 additions and 17 deletions
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207
Documentation/filesystems/vfs.txt
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@ -230,10 +230,15 @@ only called from a process context (i.e. not from an interrupt handler
or bottom half).
alloc_inode: this method is called by inode_alloc() to allocate memory
for struct inode and initialize it.
for struct inode and initialize it. If this function is not
defined, a simple 'struct inode' is allocated. Normally
alloc_inode will be used to allocate a larger structure which
contains a 'struct inode' embedded within it.
destroy_inode: this method is called by destroy_inode() to release
resources allocated for struct inode.
resources allocated for struct inode. It is only required if
->alloc_inode was defined and simply undoes anything done by
->alloc_inode.
read_inode: this method is called to read a specific inode from the
mounted filesystem. The i_ino member in the struct inode is
@ -443,14 +448,81 @@ otherwise noted.
The Address Space Object
========================
The address space object is used to identify pages in the page cache.
The address space object is used to group and manage pages in the page
cache. It can be used to keep track of the pages in a file (or
anything else) and also track the mapping of sections of the file into
process address spaces.
There are a number of distinct yet related services that an
address-space can provide. These include communicating memory
pressure, page lookup by address, and keeping track of pages tagged as
Dirty or Writeback.
The first can be used independantly to the others. The vm can try to
either write dirty pages in order to clean them, or release clean
pages in order to reuse them. To do this it can call the ->writepage
method on dirty pages, and ->releasepage on clean pages with
PagePrivate set. Clean pages without PagePrivate and with no external
references will be released without notice being given to the
address_space.
To achieve this functionality, pages need to be placed on an lru with
lru_cache_add and mark_page_active needs to be called whenever the
page is used.
Pages are normally kept in a radix tree index by ->index. This tree
maintains information about the PG_Dirty and PG_Writeback status of
each page, so that pages with either of these flags can be found
quickly.
The Dirty tag is primarily used by mpage_writepages - the default
->writepages method. It uses the tag to find dirty pages to call
->writepage on. If mpage_writepages is not used (i.e. the address
provides it's own ->writepages) , the PAGECACHE_TAG_DIRTY tag is
almost unused. write_inode_now and sync_inode do use it (through
__sync_single_inode) to check if ->writepages has been successful in
writing out the whole address_space.
The Writeback tag is used by filemap*wait* and sync_page* functions,
though wait_on_page_writeback_range, to wait for all writeback to
complete. While waiting ->sync_page (if defined) will be called on
each page that is found to require writeback
An address_space handler may attach extra information to a page,
typically using the 'private' field in the 'struct page'. If such
information is attached, the PG_Private flag should be set. This will
cause various mm routines to make extra calls into the address_space
handler to deal with that data.
An address space acts as an intermediate between storage and
application. Data is read into the address space a whole page at a
time, and provided to the application either by copying of the page,
or by memory-mapping the page.
Data is written into the address space by the application, and then
written-back to storage typically in whole pages, however the
address_space has finner control of write sizes.
The read process essentially only requires 'readpage'. The write
process is more complicated and uses prepare_write/commit_write or
set_page_dirty to write data into the address_space, and writepage,
sync_page, and writepages to writeback data to storage.
Adding and removing pages to/from an address_space is protected by the
inode's i_mutex.
When data is written to a page, the PG_Dirty flag should be set. It
typically remains set until writepage asks for it to be written. This
should clear PG_Dirty and set PG_Writeback. It can be actually
written at any point after PG_Dirty is clear. Once it is known to be
safe, PG_Writeback is cleared.
Writeback makes use of a writeback_control structure...
struct address_space_operations
-------------------------------
This describes how the VFS can manipulate mapping of a file to page cache in
your filesystem. As of kernel 2.6.13, the following members are defined:
your filesystem. As of kernel 2.6.16, the following members are defined:
struct address_space_operations {
int (*writepage)(struct page *page, struct writeback_control *wbc);
@ -469,47 +541,148 @@ struct address_space_operations {
loff_t offset, unsigned long nr_segs);
struct page* (*get_xip_page)(struct address_space *, sector_t,
int);
/* migrate the contents of a page to the specified target */
int (*migratepage) (struct page *, struct page *);
};
writepage: called by the VM write a dirty page to backing store.
writepage: called by the VM to write a dirty page to backing store.
This may happen for data integrity reason (i.e. 'sync'), or
to free up memory (flush). The difference can be seen in
wbc->sync_mode.
The PG_Dirty flag has been cleared and PageLocked is true.
writepage should start writeout, should set PG_Writeback,
and should make sure the page is unlocked, either synchronously
or asynchronously when the write operation completes.
If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
try too hard if there are problems, and may choose to write out a
different page from the mapping if that would be more
appropriate. If it chooses not to start writeout, it should
return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep
calling ->writepage on that page.
See the file "Locking" for more details.
readpage: called by the VM to read a page from backing store.
The page will be Locked when readpage is called, and should be
unlocked and marked uptodate once the read completes.
If ->readpage discovers that it needs to unlock the page for
some reason, it can do so, and then return AOP_TRUNCATED_PAGE.
In this case, the page will be re-located, re-locked and if
that all succeeds, ->readpage will be called again.
sync_page: called by the VM to notify the backing store to perform all
queued I/O operations for a page. I/O operations for other pages
associated with this address_space object may also be performed.
This function is optional and is called only for pages with
PG_Writeback set while waiting for the writeback to complete.
writepages: called by the VM to write out pages associated with the
address_space object.
address_space object. If WBC_SYNC_ALL, then the
writeback_control will specify a range of pages that must be
written out. If WBC_SYNC_NONE, then a nr_to_write is given
and that many pages should be written if possible.
If no ->writepages is given, then mpage_writepages is used
instead. This will choose pages from the addresspace that are
tagged as DIRTY and will pass them to ->writepage.
set_page_dirty: called by the VM to set a page dirty.
This is particularly needed if an address space attaches
private data to a page, and that data needs to be updated when
a page is dirtied. This is called, for example, when a memory
mapped page gets modified.
If defined, it should set the PageDirty flag, and the
PAGECACHE_TAG_DIRTY tag in the radix tree.
readpages: called by the VM to read pages associated with the address_space
object.
object. This is essentially just a vector version of
readpage. Instead of just one page, several pages are
requested.
readpages is only used for readahead, so read errors are
ignored. If anything goes wrong, feel free to give up.
prepare_write: called by the generic write path in VM to set up a write
request for a page.
request for a page. This indicates to the address space that
the given range of bytes are about to be written. The
address_space should check that the write will be able to
complete, by allocating space if necessary and doing any other
internal house keeping. If the write will update parts of
any basic-blocks on storage, then those blocks should be
pre-read (if they haven't been read already) so that the
updated blocks can be written out properly.
The page will be locked. If prepare_write wants to unlock the
page it, like readpage, may do so and return
AOP_TRUNCATED_PAGE.
In this case the prepare_write will be retried one the lock is
regained.
commit_write: called by the generic write path in VM to write page to
its backing store.
commit_write: If prepare_write succeeds, new data will be copied
into the page and then commit_write will be called. It will
typically update the size of the file (if appropriate) and
mark the inode as dirty, and do any other related housekeeping
operations. It should avoid returning an error if possible -
errors should have been handled by prepare_write.
bmap: called by the VFS to map a logical block offset within object to
physical block number. This method is use by for the legacy FIBMAP
ioctl. Other uses are discouraged.
physical block number. This method is used by for the FIBMAP
ioctl and for working with swap-files. To be able to swap to
a file, the file must have as stable mapping to a block
device. The swap system does not go through the filesystem
but instead uses bmap to find out where the blocks in the file
are and uses those addresses directly.
invalidatepage: called by the VM on truncate to disassociate a page from its
address_space mapping.
releasepage: called by the VFS to release filesystem specific metadata from
a page.
invalidatepage: If a page has PagePrivate set, then invalidatepage
will be called when part or all of the page is to be removed
from the address space. This generally corresponds either a
truncation or a complete invalidation of the address space
(in the latter case 'offset' will always be 0).
Any private data associated with the page should be updated
to reflect this truncation. If offset is 0, then
the private data should be released, because the page
must be able to be completely discarded. This may be done by
calling the ->releasepage function, but in this case the
release MUST succeed.
direct_IO: called by the VM for direct I/O writes and reads.
releasepage: releasepage is called on PagePrivate pages to indicate
that the page should be freed if possible. ->releasepage
should remove any private data from the page and clear the
PagePrivate flag. It may also remove the page from the
address_space. If this fails for some reason, it may indicate
failure with a 0 return value.
This is used in two distinct though related cases. The first
is when the VM finds a clean page with no active users and
wants to make it a free page. If ->releasepage succeeds, the
page will be removed from the address_space and become free.
The second case if when a request has been made to invalidate
some or all pages in an address_space. This can happen
through the fadvice(POSIX_FADV_DONTNEED) system call or by the
filesystem explicitly requesting it as nfs and 9fs do (when
they believe the cache may be out of date with storage) by
calling invalidate_inode_pages2().
If the filesystem makes such a call, and needs to be certain
that all pages are invalidated, then it's releasepage will
need to ensure this. Possibly it can clear the PageUptodate
bit if it cannot free private data yet.
direct_IO: called by the generic read/write routines to perform
direct_IO - that is IO requests which bypass the page cache
and tranfer data directly between the storage and the
application's address space.
get_xip_page: called by the VM to translate a block number to a page.
The page is valid until the corresponding filesystem is unmounted.
Filesystems that want to use execute-in-place (XIP) need to implement
it. An example implementation can be found in fs/ext2/xip.c.
migrate_page: This is used to compact the physical memory usage.
If the VM wants to relocate a page (maybe off a memory card
that is signalling imminent failure) it will pass a new page
and an old page to this function. migrate_page should
transfer any private data across and update any references
that it has to the page.
The File Object
===============