linux-kernel - [PATCH] mprotect: drop overprotective lock_pte

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Date:   Tue,  7 Feb 2017 17:33:47 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Andrea Arcangeli <aarcange@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: [PATCH] mprotect: drop overprotective lock_pte_protection()

lock_pte_protection() uses pmd_lock() to make sure that we have stable
PTE page table before walking pte range.

That's not necessary. We only need to make sure that PTE page table is
established. It cannot vanish under us as long as we hold mmap_sem at
least for read.

And we already have helper for that -- pmd_trans_unstable().

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
---
 mm/mprotect.c | 43 ++++++++++++-------------------------------
 1 file changed, 12 insertions(+), 31 deletions(-)

diff --git a/mm/mprotect.c b/mm/mprotect.c
index f9c07f54dd62..e919e4613eab 100644
--- 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 +43,7 @@ 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);
+	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
 	if (!pte)
 		return 0;
 
@@ -177,8 +149,6 @@ 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);
@@ -195,6 +165,17 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
 			}
 			/* fall through, the trans huge pmd just split */
 		}
+
+		/*
+		 * For prot_numa update we only hold mmap_sem for read so there
+		 * is a potential race with faulting where a pmd was
+		 * temporarily none.
+		 * Make sure we have PTE page table, before moving forward.
+		 * Page tables cannot go away under us as long as we hold
+		 * mmap_sem at least for read.
+		 */
+		if (pmd_trans_unstable(pmd))
+			continue;
 		this_pages = change_pte_range(vma, pmd, addr, next, newprot,
 				 dirty_accountable, prot_numa);
 		pages += this_pages;
-- 
2.11.0