Re: [PATCH] mm: vmscan: unlock_page page when forcing reclaim

[Vlastimil Babka <vbabka@suse.cz>]

On 07/18/2014 08:51 PM, Richard Yao wrote:
> On 07/18/2014 12:38 PM, Johannes Weiner wrote:
>> I don't really understand how the scenario you describe can happen.
>>
>> Successfully reclaiming a page means that __remove_mapping() was able
>> to freeze a page count of 2 (page cache and LRU isolation), but
>> filemap_fault() increases the refcount on the page before trying to
>> lock the page.  If __remove_mapping() wins, find_get_page() does not
>> work and the fault does not lock the page.  If find_get_page() wins,
>> __remove_mapping() does not work and the reclaimer aborts and does a
>> regular unlock_page().
>>
>> page_check_references() is purely about reclaim strategy, it should
>> not be essential for correctness.
>>
>
> You are right that something else is happened here. I had not spotted
> the cmpxchg being done in __remove_mapping(). If I spot something that
> looks like it could be what went wrong doing this, I will propose a new
> fix to the list for review. Thanks for your time.
>
> P.S. The system had ECC RAM, so this was not a bit flip. My current
> method for debugging this involves using cscope to construct possible
> call paths under a couple of assumptions:
>
> 1. Something set PG_locked without calling unlock_page().
> 2. The only ways of doing #1 that I see in the code are calling
> __clear_page_locked() or failing to clear the bit. I do not believe that
> a patch was accepted that did the latter, so I assume the former.

Could it be that the process holding the lock was also stuck doing 
something, and it was not a missed unlock?

> I have root access to the system, so each time I do a lookup using
> cscope, I go through the list to logically eliminate possibilities by
> inspecting the system where the problem occurred. When I cannot
> eliminate a possibility, I recurse. This is prone to fail positives
> should I miss a subtle piece of code that prevents a problem and it is
> very tedious, but I do not see a better way of debugging based on what I
> have at my disposal. If anyone has any suggestions, I would appreciate them.

You could try enabling VM_DEBUG, possibly LOCKDEP, try a git bisect if 
there's a previous known working kernel version...

> P.P.S. I *really* wish that I had used kdump when this issue happened,
> but sadly, the system is not setup for kdump.

So it happened only once so far? How about enabling kdump and waiting if 
it happens again.