Re: [PATCH v3 09/17] mm, compaction: make whole_zone flag ignore cached scanner positions

[Joonsoo Kim]

On Mon, Jul 18, 2016 at 11:12:51AM +0200, Vlastimil Babka wrote:
> On 07/06/2016 07:09 AM, Joonsoo Kim wrote:
> >On Fri, Jun 24, 2016 at 11:54:29AM +0200, Vlastimil Babka wrote:
> >>A recent patch has added whole_zone flag that compaction sets when scanning
> >>starts from the zone boundary, in order to report that zone has been fully
> >>scanned in one attempt. For allocations that want to try really hard or cannot
> >>fail, we will want to introduce a mode where scanning whole zone is guaranteed
> >>regardless of the cached positions.
> >>
> >>This patch reuses the whole_zone flag in a way that if it's already passed true
> >>to compaction, the cached scanner positions are ignored. Employing this flag
> >
> >Okay. But, please don't reset cached scanner position even if whole_zone
> >flag is set. Just set cc->migrate_pfn and free_pfn, appropriately. With
> 
> Won't that result in confusion on cached position updates during
> compaction where it checks the previous cached position? I wonder
> what kinds of corner cases it can bring...

whole_zone would come along with ignore_skip_hint so I think that
there is no problem on cached position updating.

> 
> >your following patches, whole_zone could be set without any compaction
> >try
> 
> I don't understand what you mean here? Even after whole series,
> whole_zone is only checked, and positions thus reset, after passing
> the compaction_suitable() call from compact_zone(). So at that point
> we can say that compaction is being actually tried and it's not a
> drive-by reset?

My point is that we should not initialize zone's cached pfn in case of
the whole_zone because what compaction with COMPACT_PRIO_SYNC_FULL
want is just to scan whole range. zone's cached pfn exists for
efficiency and there is no reason to initialize it by compaction with
COMPACT_PRIO_SYNC_FULL. If there are some parallel compaction users,
they could be benefit from un-initialized zone's cached pfn so I'd
like to leave them.

Thanks.