Re: [PATCH 12/14] mm, oom: protect !costly allocations some more

[Joonsoo Kim]

2016-05-04 17:56 GMT+09:00 Michal Hocko <mhocko@xxxxxxxxxx>:
> On Wed 04-05-16 15:31:12, Joonsoo Kim wrote:
>> On Wed, May 04, 2016 at 03:01:24PM +0900, Joonsoo Kim wrote:
>> > On Wed, Apr 20, 2016 at 03:47:25PM -0400, Michal Hocko wrote:
> [...]
>> > > @@ -3408,6 +3456,17 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>> > >                            no_progress_loops))
>> > >           goto retry;
>> > >
>> > > + /*
>> > > +  * It doesn't make any sense to retry for the compaction if the order-0
>> > > +  * reclaim is not able to make any progress because the current
>> > > +  * implementation of the compaction depends on the sufficient amount
>> > > +  * of free memory (see __compaction_suitable)
>> > > +  */
>> > > + if (did_some_progress > 0 &&
>> > > +                 should_compact_retry(order, compact_result,
>> > > +                         &migration_mode, compaction_retries))
>> >
>> > Checking did_some_progress on each round have subtle corner case. Think
>> > about following situation.
>> >
>> > round, compaction, did_some_progress, compaction
>> > 0, defer, 1
>> > 0, defer, 1
>> > 0, defer, 1
>> > 0, defer, 1
>> > 0, defer, 0
>>
>> Oops...Example should be below one.
>>
>> 0, defer, 1
>> 1, defer, 1
>> 2, defer, 1
>> 3, defer, 1
>> 4, defer, 0
>
> I am not sure I understand. The point of the check is that if the
> reclaim doesn't make _any_ progress then checking the result of the
> compaction after it didn't lead to a successful allocation just doesn't
> make any sense.

Even if this round (#4) doesn't reclaim any pages, previous rounds
(#0, #1, #2, #3) would reclaim enough pages to succeed future
compaction attempt.

Thanks.