linux-kernel - Re: [Question] ksm: rmap_item pointing to some stale vmas

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Message-ID: <alpine.LSU.2.11.1504101047200.28925@eggly.anvils>
Date:	Fri, 10 Apr 2015 10:56:07 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Susheel Khiani <skhiani@...eaurora.org>
cc:	akpm@...ux-foundation.org, peterz@...radead.org, neilb@...e.de,
	dhowells@...hat.com, hughd@...gle.com, paulmcquad@...il.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [Question] ksm: rmap_item pointing to some stale vmas

On Thu, 9 Apr 2015, Susheel Khiani wrote:

> Hi,
> 
> We are seeing an issue during try_to_unmap_ksm where in call to
> try_to_unmap_one is failing.
> 
> try_to_unmap_ksm in this particular case is trying to go through vmas
> associated with each rmap_item->anon_vma. What we see is this that the
> corresponding page is not mapped to any of the vmas associated with 2
> rmap_item.
> 
> The associated rmap_item in this case looks like pointing to some valid vma
> but the said page is not found to be mapped under it. try_to_unmap_one thus
> fails to find valid ptes for these vmas.
> 
> At the same time we can see that the page actually is mapped in 2 separate
> and different vmas which are not part of rmap_item associated with page.
> 
> So whether rmap_item is pointing to some stale vmas and now the mapping has
> changed? Or there is something else going on here.
> p
> Any pointer would be appreciated.

I expected to be able to argue this away, but no: I think you've found
a bug, and I think I get it too.  I have no idea what's wrong at this
point, will set aside some time to investigate, and report back.

Which kernel are you using?  try_to_unmap_ksm says v3.13 or earlier.
Probably doesn't affect the bug, but may affect the patch you'll need.

Hugh
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