Messages in this thread Patch in this message | | | Date | Mon, 12 Apr 2010 18:08:04 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [PATCH 4/4] anonvma: when setting up page->mapping, we need to pick the _oldest_ anonvma |
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On Tue, 13 Apr 2010, Johannes Weiner wrote: > > Would you mind pasting that nice description of the error case from your > other email into that changelog? I skimmed over the description but when > I read this patch several hours later, I had to go back to that previous > email to fully make sense of it.
It now looks like this..
Linus --- 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.
Here's the scenario:
- page gets allocated/mapped by process A. Let's call the anon_vma we associate the page with 'A' to keep it easy to track.
- Process A forks, creating process B. The anon_vma in B is 'B', and has a chain that looks like 'B' -> 'A'. Everything is fine.
- Swapping happens. The page (with mapping pointing to 'A') gets swapped out (perhaps not to disk - it's enough to assume that it's just not mapped any more, and lives entirely in the swap-cache)
- Process B pages it in, which goes like this:
do_swap_page -> page = lookup_swap_cache(entry); ... set_pte_at(mm, address, page_table, pte); page_add_anon_rmap(page, vma, address);
And think about what happens here!
In particular, what happens is that this will now be the "first" mapping of that page, so page_add_anon_rmap() used to do
if (first) __page_set_anon_rmap(page, vma, address);
and notice what anon_vma it will use? It will use the anon_vma for process B!
What happens then? Trivial: process 'A' also pages it in (nothing happens, it's not the first mapping), and then process 'B' execve's or exits or unmaps, making anon_vma B go away.
End result: process A has a page that points to anon_vma B, but anon_vma B does not exist any more. This can go on forever. Forget about RCU grace periods, forget about locking, forget anything like that. The bug is simply that page->mapping points to an anon_vma that was correct at one point, but was _not_ the one that was shared by all users of that possible mapping.
Changing it to always use the deepest anon_vma in the anonvma chain gets us to the safest model.
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.
Reviewed-by: Rik van Riel <riel@redhat.com> Acked-by: Johannes Weiner <hannes@cmpxchg.org> Tested-by: Borislav Petkov <bp@alien8.de> [ "What do you know, I think you fixed it!" ] 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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