Re: [RFC 12/13] mm, compaction: more reliably increase direct compaction priority

Vlastimil Babka
2016-05-31T12:29:29Z

On 05/31/2016 02:07 PM, Vlastimil Babka wrote:
> On 05/31/2016 08:37 AM, Joonsoo Kim wrote:
>>> @@ -3695,22 +3695,22 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>>> else
>>> no_progress_loops++;
>>>
>>> - if (should_reclaim_retry(gfp_mask, order, ac, alloc_flags,
>>> - did_some_progress > 0, no_progress_loops))
>>> - goto retry;
>>> -
>>> + should_retry = should_reclaim_retry(gfp_mask, order, ac, alloc_flags,
>>> + did_some_progress > 0, no_progress_loops);
>>> /*
>>> * 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(ac, order, alloc_flags,
>>> + if (did_some_progress > 0)
>>> + should_retry |= should_compact_retry(ac, order, alloc_flags,
>>> compact_result, &compact_priority,
>>> - compaction_retries))
>>> + compaction_retries);
>>> + if (should_retry)
>>> goto retry;
>>
>> Hmm... it looks odd that we check should_compact_retry() when
>> did_some_progress > 0. If system is full of anonymous memory and we
>> don't have swap, we can't reclaim anything but we can compact.
>
> Right, thanks.

Hmm on the other hand, should_compact_retry will assume (in
compaction_zonelist_suitable()) that reclaimable memory is actually
reclaimable. If there's nothing to tell us that it actually isn't, if we
drop the reclaim progress requirement. That's risking an infinite loop?