Re: [PATCH -v2] rmap: make anon_vma_prepare link in all theanon_vmas of a mergeable VMA

[Borislav Petkov]

From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
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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