[PATCH] mm: protect against concurrent vma expansion

[Michel Lespinasse]

expand_stack() runs with a shared mmap_sem lock. Because of this, there
could be multiple concurrent stack expansions in the same mm, which may
cause problems in the vma gap update code.

I propose to solve this by taking the mm->page_table_lock around such vma
expansions, in order to avoid the concurrency issue. We only have to worry
about concurrent expand_stack() calls here, since we hold a shared mmap_sem
lock and all vma modificaitons other than expand_stack() are done under
an exclusive mmap_sem lock.

I previously tried to achieve the same effect by making sure all
growable vmas in a given mm would share the same anon_vma, which we
already lock here. However this turned out to be difficult - all of the
schemes I tried for refcounting the growable anon_vma and clearing
turned out ugly. So, I'm now proposing only the minimal fix.

Signed-off-by: Michel Lespinasse <walken@xxxxxxxxxx>

---
 mm/mmap.c |   14 ++++++++++++++
 1 files changed, 14 insertions(+), 0 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 9ed3a06242a0..e44fe876a7e3 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2069,6 +2069,11 @@ int expand_upwards(struct vm_area_struct *vma, unsigned long address)
 		if (vma->vm_pgoff + (size >> PAGE_SHIFT) >= vma->vm_pgoff) {
 			error = acct_stack_growth(vma, size, grow);
 			if (!error) {
+				/*
+				 * page_table_lock to protect against
+				 * concurrent vma expansions
+				 */
+				spin_lock(&vma->vm_mm->page_table_lock);
 				anon_vma_interval_tree_pre_update_vma(vma);
 				vma->vm_end = address;
 				anon_vma_interval_tree_post_update_vma(vma);
@@ -2076,6 +2081,8 @@ int expand_upwards(struct vm_area_struct *vma, unsigned long address)
 					vma_gap_update(vma->vm_next);
 				else
 					vma->vm_mm->highest_vm_end = address;
+				spin_unlock(&vma->vm_mm->page_table_lock);
+
 				perf_event_mmap(vma);
 			}
 		}
@@ -2126,11 +2133,18 @@ int expand_downwards(struct vm_area_struct *vma,
 		if (grow <= vma->vm_pgoff) {
 			error = acct_stack_growth(vma, size, grow);
 			if (!error) {
+				/*
+				 * page_table_lock to protect against
+				 * concurrent vma expansions
+				 */
+				spin_lock(&vma->vm_mm->page_table_lock);
 				anon_vma_interval_tree_pre_update_vma(vma);
 				vma->vm_start = address;
 				vma->vm_pgoff -= grow;
 				anon_vma_interval_tree_post_update_vma(vma);
 				vma_gap_update(vma);
+				spin_unlock(&vma->vm_mm->page_table_lock);
+
 				perf_event_mmap(vma);
 			}
 		}
-- 
1.7.7.3
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