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

[Vlastimil Babka]

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...

> 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?

Thanks