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

[Susheel Khiani <skhiani@codeaurora.org>]

On 04/10/15 23:26, Hugh Dickins wrote:
> 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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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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