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Subject[PATCH 4/4] anonvma: when setting up page->mapping, we need to pick the _oldest_ anonvma

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 12 Apr 2010 12:44:29 -0700
Subject: [PATCH 4/4] anonvma: when setting up page->mapping, we need to pick the _oldest_ anonvma

Otherwise we might be mapping in a page in a new mapping, but that page
(through the swapcache) would later be mapped into an old mapping too.
The page->mapping must be the case that works for everybody, not just
the mapping that happened to page it in first.

This can be improved in certain cases: if we know the page is private to
just this particular mapping (for example, it's a new page, or it is the
only swapcache entry), we could pick the top (most specific) anon_vma.

But that's a future optimization. Make it _work_ reliably first.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
mm/rmap.c | 15 +++++++++++++--
1 files changed, 13 insertions(+), 2 deletions(-)
diff --git a/mm/rmap.c b/mm/rmap.c
index ee97d38..4bad326 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -734,9 +734,20 @@ void page_move_anon_rmap(struct page *page,
static void __page_set_anon_rmap(struct page *page,
struct vm_area_struct *vma, unsigned long address)
{
- struct anon_vma *anon_vma = vma->anon_vma;
+ struct anon_vma_chain *avc;
+ struct anon_vma *anon_vma;
+
+ BUG_ON(!vma->anon_vma);
+
+ /*
+ * We must use the _oldest_ possible anon_vma for the page mapping!
+ *
+ * So take the last AVC chain entry in the vma, which is the deepest
+ * ancestor, and use the anon_vma from that.
+ */
+ avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
+ anon_vma = avc->anon_vma;

- BUG_ON(!anon_vma);
anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
page->mapping = (struct address_space *) anon_vma;
page->index = linear_page_index(vma, address);
--
1.7.1.rc1.dirty


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