Re: [PATCH v5 02/14] mm, compaction: defer each zone individually instead of preferred zone

[Vlastimil Babka]

On 07/29/2014 01:59 AM, David Rientjes wrote:
> > --- a/mm/compaction.c
> > +++ b/mm/compaction.c
> > @@ -1122,28 +1122,27 @@ int sysctl_extfrag_threshold = 500;
> >    * @nodemask: The allowed nodes to allocate from
> >    * @mode: The migration mode for async, sync light, or sync migration
> >    * @contended: Return value that is true if compaction was aborted due to lock contention
> > - * @page: Optionally capture a free page of the requested order during compaction
>
> Never noticed this non-existant formal before.

It's a leftover from the first Mel's page capture attempt that was 
partially reverted.

> > + * @candidate_zone: Return the zone where we think allocation should succeed
> >    *
> >    * This is the main entry point for direct page compaction.
> >    */
> >   unsigned long try_to_compact_pages(struct zonelist *zonelist,
> >   			int order, gfp_t gfp_mask, nodemask_t *nodemask,
> > -			enum migrate_mode mode, bool *contended)
> > +			enum migrate_mode mode, bool *contended,
> > +			struct zone **candidate_zone)
> >   {
> >   	enum zone_type high_zoneidx = gfp_zone(gfp_mask);
> >   	int may_enter_fs = gfp_mask & __GFP_FS;
> >   	int may_perform_io = gfp_mask & __GFP_IO;
> >   	struct zoneref *z;
> >   	struct zone *zone;
> > -	int rc = COMPACT_SKIPPED;
> > +	int rc = COMPACT_DEFERRED;
> >   	int alloc_flags = 0;
> >
> >   	/* Check if the GFP flags allow compaction */
> >   	if (!order || !may_enter_fs || !may_perform_io)
> >   		return rc;
>
> It doesn't seem right that if we called try_to_compact_pages() in a
> context where it is useless (order-0 or non-GFP_KERNEL allocation) that we
> would return COMPACT_DEFERRED.  I think the existing semantics before the
> patch, that is
>
>   - deferred: compaction was tried but failed, so avoid subsequent calls in
>     the near future that may be potentially expensive, and
>
>   - skipped: compaction wasn't tried because it will be useless
>
> is correct and deferred shouldn't take on another meaning, which now will
> set deferred_compaction == true in the page allocator.  It probably
> doesn't matter right now because the only check of deferred_compaction is
> effective for __GFP_NO_KSWAPD, i.e. it is both high-order and GFP_KERNEL,
> but it seems returning COMPACT_SKIPPED here would also work fine and be
> more appropriate.

You're right. That was an oversight, not intention. Thanks for catching 
that.

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