Messages in this thread | | | Date | Sat, 10 Apr 2010 20:51:45 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon_vmas of a mergeable VMA |
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From: Linus Torvalds <torvalds@linux-foundation.org> Date: Sat, Apr 10, 2010 at 11:21:39AM -0700
> On Sat, 10 Apr 2010, Linus Torvalds wrote: > > On Sat, 10 Apr 2010, Borislav Petkov wrote: > > > > > > And I got an oops again, this time the #GP from couple of days ago. > > > > Oh damn. So the list corruption really does happen still. > > Ho humm. > > Maybe I'm crazy, but something started bothering me. And I started > wondering: when is the 'page->mapping' of an anonymous page actually > cleared? > > The thing is, the mapping of an anonymous page is actually cleared only > when the page is _freed_, in "free_hot_cold_page()". > > Now, let's think about that. And in particular, let's think about how that > relates to the freeing of the 'anon_vma' that the page->mapping points to. > > The way the anon_vma is freed is when the mapping is torn down, and we do > roughly: > > tlb = tlb_gather_mmu(mm,..) > .. > unmap_vmas(&tlb, vma .. > .. > free_pgtables() > .. > tlb_finish_mmu(tlb, start, end); > > and we actually unmap all the pages in "unmap_vmas()", and then _after_ > unmapping all the pages we do the "unlink_anon_vmas(vma);" in > "free_pgtables()". Fine so far - the anon_vma stay around until after the > page has been happily unmapped. > > But "unmapped all the pages" is _not_ actually the same as "free'd all the > pages". The actual _freeing_ of the page happens generally in > tlb_finish_mmu(), because we can free the page only after we've flushed > any TLB entries. > > So what we have in that tlb_gather structure is a list of _pending_ pages > to be freed, while we already actually free'd the anon_vmas earlier! > > Now, the thing is, tlb_gather_mmu() begins a preempt-safe region (because > we use a per-cpu variable), but as far as I can tell it is _not_ an > RCU-safe region. > > So I think we might actually get a real RCU freeing event while this all > happens. So now the 'anon_vma' that 'page->mapping' points to has not just > been released back to the SLUB caches, the page itself might have been > released too.
So, if I understand you correctly, the list_head anon_vma gets freed _before_ the page descriptor itself, therefore we still get a valid page->mapping in page_lock_anon_vma(). Maybe that explains the funny patterns in %r13. But how do they come to exist when the anon_vma is freed, shouldn't there be LIST_POISON or something recognizable?
Anyways, testing...
-- Regards/Gruss, Boris.
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