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

From: Susheel Khiani
Date: Tue Apr 14 2015 - 03:01:47 EST

On 04/10/15 23:26, Hugh Dickins wrote:
On Thu, 9 Apr 2015, Susheel Khiani wrote:


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

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.
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.


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We are using kernel-3.10.49 and I have gone through patches of ksm above this kernel version but didn't find anything relevant w.r.t issue. The latest patch which we have for KSM on our tree is

668f9abb: mm: close PageTail race

The issue otherwise is difficult to reproduce and is appearing after days of testing on 512MB Android platform. What I am not able to figure out is which code path in ksm could actually land us in situation where in stable_node we still have stale rmap_items with old vmas which are now unmapped.

In the dumps we can see the new vmas mapping to the page but the new rmap_items with these new vmas which maps the page are still not updated in stable_node.

Susheel Khiani QUALCOMM INDIA, on behalf of Qualcomm Innovation Center,
Inc. is a member of the Code Aurora Forum, hosted by The Linux Foundation
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