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From: Mel Gorman on 27 Apr 2010 17:40
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 | 22 ++++++++++++++++++++--
mm/mmap.c | 6 ++++++
mm/rmap.c | 28 +++++++++++++++++++++++-----
3 files changed, 49 insertions(+), 7 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 3666d43..0c09927 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1668,15 +1668,28 @@ int rmap_walk_ksm(struct page *page, int (*rmap_one)(struct page *,
again:
hlist_for_each_entry(rmap_item, hlist, &stable_node->hlist, hlist) {
struct anon_vma *anon_vma = rmap_item->anon_vma;
+ struct anon_vma *locked_vma;
struct anon_vma_chain *vmac;
struct vm_area_struct *vma;

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 */
+ locked_vma = NULL;
+ if (anon_vma != vma->anon_vma) {
+ locked_vma = vma->anon_vma;
+ if (!spin_trylock(&locked_vma->lock)) {
+ spin_unlock(&anon_vma->lock);
+ goto again;
+ }
+ }
+
if (rmap_item->address < vma->vm_start ||
rmap_item->address >= vma->vm_end)
- continue;
+ goto next_vma;
+
/*
* Initially we examine only the vma which covers this
* rmap_item; but later, if there is still work to do,
@@ -1684,9 +1697,14 @@ again:
* were forked from the original since ksmd passed.
*/
if ((rmap_item->mm == vma->vm_mm) == search_new_forks)
- continue;
+ goto next_vma;

ret = rmap_one(page, vma, rmap_item->address, arg);
+
+next_vma:
+ if (locked_vma)
+ spin_unlock(&locked_vma->lock);
+
if (ret != SWAP_AGAIN) {
spin_unlock(&anon_vma->lock);
goto out;
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..f7ed89f 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1358,7 +1358,7 @@ int try_to_munlock(struct page *page)
static int rmap_walk_anon(struct page *page, int (*rmap_one)(struct page *,
struct vm_area_struct *, unsigned long, void *), void *arg)
{
- struct anon_vma *anon_vma;
+ struct anon_vma *anon_vma, *locked_vma;
struct anon_vma_chain *avc;
int ret = SWAP_AGAIN;

@@ -1368,16 +1368,34 @@ 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;
- ret = rmap_one(page, vma, address, arg);
+ unsigned long address;
+
+ /*
+ * Guard against deadlocks by not spinning against
+ * vma->anon_vma->lock. On contention release and retry
+ */
+ locked_vma = NULL;
+ if (anon_vma != vma->anon_vma) {
+ locked_vma = vma->anon_vma;
+ if (!spin_trylock(&locked_vma->lock)) {
+ spin_unlock(&anon_vma->lock);
+ goto retry;
+ }
+ }
+ address = vma_address(page, vma);
+ if (address != -EFAULT)
+ ret = rmap_one(page, vma, address, arg);
+
+ if (locked_vma)
+ spin_unlock(&locked_vma->lock);
+
if (ret != SWAP_AGAIN)
break;
}
--
1.6.5

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Prev: [PATCH 0/3] Fix migration races in rmap_walk() V2
Next: [RFC] ehci: Disable wake on overcurrent (WKOC_E) and disconnect (WKDISC_E)