ksm: fix page_address_in_vma anon_vma oops
2.6.36-rc1 commit 21d0d443cdc1658a8c1484fdcece4803f0f96d0e "rmap:
resurrect page_address_in_vma anon_vma check" was right to resurrect
that check; but now that it's comparing anon_vma->roots instead of
just anon_vmas, there's a danger of oopsing on a NULL anon_vma.
In most cases no NULL anon_vma ever gets here; but it turns out that
occasionally KSM, when enabled on a forked or forking process, will
itself call page_address_in_vma() on a "half-KSM" page left over from
an earlier failed attempt to merge - whose page_anon_vma() is NULL.
It's my bug that those should be getting here at all: I thought they
were already dealt with, this oops proves me wrong, I'll fix it in
the next release - such pages are effectively pinned until their
process exits, since rmap cannot find their ptes (though swapoff can).
For now just work around it by making page_address_in_vma() safe (and
add a comment on why that check is wanted anyway). A similar check
in __page_check_anon_rmap() is safe because do_page_add_anon_rmap()
already excluded KSM pages.
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/rmap.c b/mm/rmap.c
index 9d2ba01..92e6757 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -381,7 +381,13 @@
unsigned long page_address_in_vma(struct page *page, struct vm_area_struct *vma)
{
if (PageAnon(page)) {
- if (vma->anon_vma->root != page_anon_vma(page)->root)
+ struct anon_vma *page__anon_vma = page_anon_vma(page);
+ /*
+ * Note: swapoff's unuse_vma() is more efficient with this
+ * check, and needs it to match anon_vma when KSM is active.
+ */
+ if (!vma->anon_vma || !page__anon_vma ||
+ vma->anon_vma->root != page__anon_vma->root)
return -EFAULT;
} else if (page->mapping && !(vma->vm_flags & VM_NONLINEAR)) {
if (!vma->vm_file ||