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Commit 175ad4f1 authored by Andrea Arcangeli's avatar Andrea Arcangeli Committed by Linus Torvalds
Browse files

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: default avatarAndrea 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 7a0c4cf8

mm/mprotect.c

+15 −31
Original line number Original line Diff line number Diff line
@@ -33,34 +33,6 @@ 




#include "internal.h"

#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,

static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,

		unsigned long addr, unsigned long end, pgprot_t newprot,

		unsigned long addr, unsigned long end, pgprot_t newprot,

		int dirty_accountable, int prot_numa)

		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;

	unsigned long pages = 0;

	int target_node = NUMA_NO_NODE;

	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)

	if (!pte)

		return 0;

		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 (pmd_trans_huge(*pmd) || pmd_devmap(*pmd)) {

			if (next - addr != HPAGE_PMD_SIZE) {

			if (next - addr != HPAGE_PMD_SIZE) {

				__split_huge_pmd(vma, pmd, addr, false, NULL);

				__split_huge_pmd(vma, pmd, addr, false, NULL);

				if (pmd_trans_unstable(pmd))

					continue;

			} else {

			} else {

				int nr_ptes = change_huge_pmd(vma, pmd, addr,

				int nr_ptes = change_huge_pmd(vma, pmd, addr,

						newprot, prot_numa);

						newprot, prot_numa);