linux-kernel - [PATCH 1/2] mm: fix the racy mm->locked

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Date:	Tue, 29 Sep 2015 20:27:56 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Andrey Konovalov <andreyknvl@...gle.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Hugh Dickins <hughd@...gle.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Sasha Levin <sasha.levin@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] mm: fix the racy mm->locked_vm change in

"mm->locked_vm += grow" and vm_stat_account() in acct_stack_growth()
are not safe; multiple threads using the same ->mm can do this at the
same time trying to expans different vma's under down_read(mmap_sem).
This means that one of the "locked_vm += grow" changes can be lost
and we can miss munlock_vma_pages_all() later.

Move this code into the caller(s) under mm->page_table_lock. All other
updates to ->locked_vm hold mmap_sem for writing.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 mm/mmap.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 8393580..4efdc37 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2138,10 +2138,6 @@ static int acct_stack_growth(struct vm_area_struct *vma, unsigned long size, uns
 	if (security_vm_enough_memory_mm(mm, grow))
 		return -ENOMEM;
 
-	/* Ok, everything looks good - let it rip */
-	if (vma->vm_flags & VM_LOCKED)
-		mm->locked_vm += grow;
-	vm_stat_account(mm, vma->vm_flags, vma->vm_file, grow);
 	return 0;
 }
 
@@ -2202,6 +2198,10 @@ int expand_upwards(struct vm_area_struct *vma, unsigned long address)
 				 * against concurrent vma expansions.
 				 */
 				spin_lock(&vma->vm_mm->page_table_lock);
+				if (vma->vm_flags & VM_LOCKED)
+					vma->vm_mm->locked_vm += grow;
+				vm_stat_account(vma->vm_mm, vma->vm_flags,
+						vma->vm_file, grow);
 				anon_vma_interval_tree_pre_update_vma(vma);
 				vma->vm_end = address;
 				anon_vma_interval_tree_post_update_vma(vma);
@@ -2273,6 +2273,10 @@ int expand_downwards(struct vm_area_struct *vma,
 				 * against concurrent vma expansions.
 				 */
 				spin_lock(&vma->vm_mm->page_table_lock);
+				if (vma->vm_flags & VM_LOCKED)
+					vma->vm_mm->locked_vm += grow;
+				vm_stat_account(vma->vm_mm, vma->vm_flags,
+						vma->vm_file, grow);
 				anon_vma_interval_tree_pre_update_vma(vma);
 				vma->vm_start = address;
 				vma->vm_pgoff -= grow;
-- 
2.4.3

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