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>

diff --git a/mm/mprotect.c b/mm/mprotect.c
index f9c07f54dd62d928187985554695c3c1117c75a7..a45b4dc6a7f5d7577b56193d2dd2115bee2a7d59 100644 (file)
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -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);