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

[Vlastimil Babka]

On 07/19/2016 08:44 AM, Joonsoo Kim wrote:
> 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.

Right, that's true.

> > > 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.

I doubt they will benefit much, but OK, I'll update the patch.

> Thanks.