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From: Mel Gorman on 26 Apr 2010 18:40
vma_adjust() is updating anon VMA information without any locks taken.
In contrast, file-backed mappings use the i_mmap_lock and this lack of
locking can result in races with page migration. During rmap_walk(),
vma_address() can return -EFAULT for an address that will soon be valid.
This leaves a dangling migration PTE behind which can later cause a BUG_ON
to trigger when the page is faulted in.

With the recent anon_vma changes, there can be more than one anon_vma->lock
that can be taken in a anon_vma_chain but a second lock cannot be spinned
upon in case of deadlock. Instead, the rmap walker tries to take locks of
different anon_vma's. If the attempt fails, the operation is restarted.

Signed-off-by: Mel Gorman <mel(a)csn.ul.ie>
---
mm/ksm.c | 13 +++++++++++++
mm/mmap.c | 6 ++++++
mm/rmap.c | 22 +++++++++++++++++++---
3 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 3666d43..baa5b4d 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1674,9 +1674,22 @@ again:
spin_lock(&anon_vma->lock);
list_for_each_entry(vmac, &anon_vma->head, same_anon_vma) {
vma = vmac->vma;
+
+ /* See comment in mm/rmap.c#rmap_walk_anon on locking */
+ if (anon_vma != vma->anon_vma) {
+ if (!spin_trylock(&vma->anon_vma->lock)) {
+ spin_unlock(&anon_vma->lock);
+ goto again;
+ }
+ }
+
if (rmap_item->address < vma->vm_start ||
rmap_item->address >= vma->vm_end)
continue;
+
+ if (anon_vma != vma->anon_vma)
+ spin_unlock(&vma->anon_vma->lock);
+
/*
* Initially we examine only the vma which covers this
* rmap_item; but later, if there is still work to do,
diff --git a/mm/mmap.c b/mm/mmap.c
index f90ea92..61d6f1d 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -578,6 +578,9 @@ again: remove_next = 1 + (end > next->vm_end);
}
}

+ if (vma->anon_vma)
+ spin_lock(&vma->anon_vma->lock);
+
if (root) {
flush_dcache_mmap_lock(mapping);
vma_prio_tree_remove(vma, root);
@@ -620,6 +623,9 @@ again: remove_next = 1 + (end > next->vm_end);
if (mapping)
spin_unlock(&mapping->i_mmap_lock);

+ if (vma->anon_vma)
+ spin_unlock(&vma->anon_vma->lock);
+
if (remove_next) {
if (file) {
fput(file);
diff --git a/mm/rmap.c b/mm/rmap.c
index 85f203e..bc313a6 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1368,15 +1368,31 @@ static int rmap_walk_anon(struct page *page, int (*rmap_one)(struct page *,
* are holding mmap_sem. Users without mmap_sem are required to
* take a reference count to prevent the anon_vma disappearing
*/
+retry:
anon_vma = page_anon_vma(page);
if (!anon_vma)
return ret;
spin_lock(&anon_vma->lock);
list_for_each_entry(avc, &anon_vma->head, same_anon_vma) {
struct vm_area_struct *vma = avc->vma;
- unsigned long address = vma_address(page, vma);
- if (address == -EFAULT)
- continue;
+ unsigned long address;
+
+ /*
+ * Guard against deadlocks by not spinning against
+ * vma->anon_vma->lock. If contention is found, release our
+ * lock and try again until VMA list can be traversed without
+ * contention.
+ */
+ if (anon_vma != vma->anon_vma) {
+ if (!spin_trylock(&vma->anon_vma->lock)) {
+ spin_unlock(&anon_vma->lock);
+ goto retry;
+ }
+ }
+ address = vma_address(page, vma);
+ if (anon_vma != vma->anon_vma)
+ spin_unlock(&vma->anon_vma->lock);
+
ret = rmap_one(page, vma, address, arg);
if (ret != SWAP_AGAIN)
break;
--
1.6.5

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Prev: [PATCH 4/6] staging: push down BKL into ioctl functions
Next: [PATCH 0/2] Fix migration races in rmap_walk()