On 03/08/2016 10:46 AM, Michal Hocko wrote:
> On Tue 08-03-16 10:24:56, Vlastimil Babka wrote:
> [...]
>>> @@ -2819,28 +2819,22 @@ static struct page *
>>> __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
>>> int alloc_flags, const struct alloc_context *ac,
>>> enum migrate_mode mode, int *contended_compaction,
>>> - bool *deferred_compaction)
>>> + unsigned long *compact_result)
>>> {
>>> - unsigned long compact_result;
>>> struct page *page;
>>>
>>> - if (!order)
>>> + if (!order) {
>>> + *compact_result = COMPACT_NONE;
>>> return NULL;
>>> + }
>>>
>>> current->flags |= PF_MEMALLOC;
>>> - compact_result = try_to_compact_pages(gfp_mask, order, alloc_flags, ac,
>>> + *compact_result = try_to_compact_pages(gfp_mask, order, alloc_flags, ac,
>>> mode, contended_compaction);
>>> current->flags &= ~PF_MEMALLOC;
>>>
>>> - switch (compact_result) {
>>> - case COMPACT_DEFERRED:
>>> - *deferred_compaction = true;
>>> - /* fall-through */
>>> - case COMPACT_SKIPPED:
>>> + if (*compact_result <= COMPACT_SKIPPED)
>>
>> COMPACT_NONE is -1 and compact_result is unsigned long, so this won't
>> work as expected.
>
> Well, COMPACT_NONE is documented as /* compaction disabled */ so we
> should never get it from try_to_compact_pages.
Right.
>
> [...]
>>> @@ -3294,6 +3289,18 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int
>>> order,
>>> did_some_progress > 0, no_progress_loops))
>>> goto retry;
>>>
>>> + /*
>>> + * !costly allocations are really important and we have to make sure
>>> + * the compaction wasn't deferred or didn't bail out early due to locks
>>> + * contention before we go OOM.
>>> + */
>>> + if (order && order <= PAGE_ALLOC_COSTLY_ORDER) {
>>> + if (compact_result <= COMPACT_CONTINUE)
>>
>> Same here.
>> I was going to say that this didn't have effect on Sergey's test, but
>> turns out it did :)
>
> This should work as expected because compact_result is unsigned long
> and so this is the unsigned arithmetic. I can make
> #define COMPACT_NONE -1UL
>
> to make the intention more obvious if you prefer, though.
Well, what wasn't obvious to me is actually that here (unlike in the
test above) it was actually intended that COMPACT_NONE doesn't result in
a retry. But it makes sense, otherwise we would retry endlessly if
reclaim couldn't form a higher-order page, right.
> Thanks for the review.
>
