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mm: mprotect: use pmd_trans_unstable instead of taking the pmd_lock 

pmd_trans_unstable does an atomic read on the pmd so it doesn't require
the pmd_lock for the same check.

This also removes the special assumption that the mmap_sem is hold for
writing if prot_numa is not set.  userfaultfd will hold the mmap_sem
only for reading in change_pte_range like prot_numa, but it will not set
prot_numa.

This is always a valid micro-optimization regardless of userfaultfd.

[kirill@shutemov.name: drop unneeded pmd_trans_unstable(pmd) check after __split_huge_pmd()]
  Link: http://lkml.kernel.org/r/20170208120421.GE5578@node.shutemov.name
Link: http://lkml.kernel.org/r/20161216144821.5183-43-aarcange@redhat.com
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Michael Rapoport <RAPOPORT@il.ibm.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Andrea Arcangeli 2017-02-22 15:44:12 -08:00 committed by Linus Torvalds
parent 7a0c4cf85b
commit 175ad4f1e7
1 changed files with 15 additions and 31 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

46
mm/mprotect.c
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@ -33,34 +33,6 @@
#include "internal.h"
/*
* For a prot_numa update we only hold mmap_sem for read so there is a
* potential race with faulting where a pmd was temporarily none. This
* function checks for a transhuge pmd under the appropriate lock. It
* returns a pte if it was successfully locked or NULL if it raced with
* a transhuge insertion.
*/
static pte_t *lock_pte_protection(struct vm_area_struct *vma, pmd_t *pmd,
unsigned long addr, int prot_numa, spinlock_t **ptl)
{
pte_t *pte;
spinlock_t *pmdl;
/* !prot_numa is protected by mmap_sem held for write */
if (!prot_numa)
return pte_offset_map_lock(vma->vm_mm, pmd, addr, ptl);
pmdl = pmd_lock(vma->vm_mm, pmd);
if (unlikely(pmd_trans_huge(*pmd) || pmd_none(*pmd))) {
spin_unlock(pmdl);
return NULL;
}
pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, ptl);
spin_unlock(pmdl);
return pte;
}
static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
unsigned long addr, unsigned long end, pgprot_t newprot,
int dirty_accountable, int prot_numa)
@ -71,7 +43,21 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
unsigned long pages = 0;
int target_node = NUMA_NO_NODE;
pte = lock_pte_protection(vma, pmd, addr, prot_numa, &ptl);
/*
* Can be called with only the mmap_sem for reading by
* prot_numa so we must check the pmd isn't constantly
* changing from under us from pmd_none to pmd_trans_huge
* and/or the other way around.
*/
if (pmd_trans_unstable(pmd))
return 0;
/*
* The pmd points to a regular pte so the pmd can't change
* from under us even if the mmap_sem is only hold for
* reading.
*/
pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
if (!pte)
return 0;
@ -177,8 +163,6 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd)) {
if (next - addr != HPAGE_PMD_SIZE) {
__split_huge_pmd(vma, pmd, addr, false, NULL);
if (pmd_trans_unstable(pmd))
continue;
} else {
int nr_ptes = change_huge_pmd(vma, pmd, addr,
newprot, prot_numa);