Re: [PATCH] mm: fix draining remote pageset

[Huang, Ying]

Michal Hocko <mhocko@xxxxxxxx> writes:

> On Mon 14-08-23 09:59:51, Huang, Ying wrote:
>> Hi, Michal,
>> 
>> Michal Hocko <mhocko@xxxxxxxx> writes:
>> 
>> > On Fri 11-08-23 17:08:19, Huang Ying wrote:
>> >> If there is no memory allocation/freeing in the remote pageset after
>> >> some time (3 seconds for now), the remote pageset will be drained to
>> >> avoid memory wastage.
>> >> 
>> >> But in the current implementation, vmstat updater worker may not be
>> >> re-queued when we are waiting for the timeout (pcp->expire != 0) if
>> >> there are no vmstat changes, for example, when CPU goes idle.
>> >
>> > Why is that a problem?
>> 
>> The pages of the remote zone may be kept in the local per-CPU pageset
>> for long time as long as there's no page allocation/freeing on the
>> logical CPU.  In addition to the logical CPU goes idle, this is also
>> possible if the logical CPU is busy in the user space.
>
> But why is this a problem? Is the scale of the problem sufficient to
> trigger out of memory situations or be otherwise harmful?

This may trigger premature page reclaiming.  The pages in the PCP of the
remote zone would have been freed to satisfy the page allocation for the
remote zone to avoid page reclaiming.  It's highly possible that the
local CPU just allocate/free from/to the remote zone temporarily.  So,
we should free PCP pages of the remote zone if there is no page
allocation/freeing from/to the remote zone for 3 seconds.

This will not trigger OOM, because all PCP will be drained if allocation
failed after direct reclaiming.

--
Best Regards,
Huang, Ying