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From: Naoya Horiguchi on 22 Apr 2010 01:50
I found a bug on page_address_in_vma() related to anon_vma_chain.

I wrote a patch, but according to a comment in include/linux/rmap.h,
I suspect this doesn't meet lock requirement of anon_vma_chain
(mmap_sem and page_table_lock, see below).

mmap_sem page_table_lock
mm/ksm.c:
write_protect_page() hold not hold
replace_page() hold not hold
mm/memory-failure.c:
add_to_kill() not hold hold
mm/mempolicy.c:
new_vma_page() hold not hold
mm/swapfile.c:
unuse_vma() hold not hold

Any comments?

Thanks,
Naoya Horiguchi
---
Subject: [BUG] rmap: fix page_address_in_vma() to walk through anon_vma_chain

page_address_in_vma() checks if a given page is associated with a given vma.
Currently it just compares vma->anon_vma and page_anon_vma(page).
But in 2.6.34, a vma can have multiple anon_vmas with anon_vma_chain,
so we have to check all anon_vmas in the "same_vma" chain.
Otherwise, when a page is shared by multiple processes,
some (page,vma) pairs can be misjudged as not-mapped.

Need Work: Meet lock requirement of anon_vma_chain.

Signed-off-by: Naoya Horiguchi <n-horiguchi(a)ah.jp.nec.com>
Cc: Andrew Morton <akpm(a)linux-foundation.org>
Cc: Rik van Riel <riel(a)redhat.com>
Cc: Andi Kleen <andi(a)firstfloor.org>
---
mm/rmap.c | 12 ++++++++++--
1 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 526704e..2e7462b 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -340,14 +340,22 @@ vma_address(struct page *page, struct vm_area_struct *vma)
unsigned long page_address_in_vma(struct page *page, struct vm_area_struct *vma)
{
if (PageAnon(page)) {
- if (vma->anon_vma != page_anon_vma(page))
- return -EFAULT;
+ struct anon_vma_chain *avc;
+ /*
+ * Walking same_vma list needs mmap_sem and page_table_lock.
+ * Do users of this function meet it?
+ */
+ list_for_each_entry(avc, &vma->anon_vma_chain, same_vma)
+ if (avc->anon_vma == page_anon_vma(page))
+ goto get_address;
+ return -EFAULT;
} else if (page->mapping && !(vma->vm_flags & VM_NONLINEAR)) {
if (!vma->vm_file ||
vma->vm_file->f_mapping != page->mapping)
return -EFAULT;
} else
return -EFAULT;
+get_address:
return vma_address(page, vma);
}

--
1.7.0
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