Re: [PATCH v2 1/5] mm: Introduce zone_appears_fragmented()

[Matthew Brost]

On Wed, Apr 22, 2026 at 11:16:37PM -0700, Matthew Brost wrote:
> On Thu, Apr 23, 2026 at 04:04:32PM +1000, Balbir Singh wrote:
> > On 4/23/26 15:56, Matthew Brost wrote:
> > > Introduce zone_appears_fragmented() as a lightweight helper to allow
> > > subsystems to make coarse decisions about reclaim behavior in the
> > > presence of likely fragmentation.
> > > 
> > > The helper implements a simple heuristic: if the number of free pages
> > > in a zone exceeds twice the high watermark, the zone is considered to
> > > have ample free memory and allocation failures are more likely due to
> > > fragmentation than overall memory pressure.
> > > 
> > > This is intentionally imprecise and is not meant to replace the core
> > > MM compaction or fragmentation accounting logic. Instead, it provides
> > > a cheap signal for callers (e.g., shrinkers) that wish to avoid
> > > overly aggressive reclaim when sufficient free memory exists but
> > > high-order allocations may still fail.
> > > 
> > > No functional changes; this is a preparatory helper for future users.
> > > 
> > > Cc: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
> > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > Cc: David Hildenbrand <david@xxxxxxxxxx>
> > > Cc: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> > > Cc: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
> > > Cc: Vlastimil Babka <vbabka@xxxxxxxxxx>
> > > Cc: Mike Rapoport <rppt@xxxxxxxxxx>
> > > Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > > Cc: linux-mm@xxxxxxxxx
> > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
> > > ---
> > >  include/linux/vmstat.h | 13 +++++++++++++
> > >  1 file changed, 13 insertions(+)
> > > 
> > > diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
> > > index 3c9c266cf782..568d9f4f1a1f 100644
> > > --- a/include/linux/vmstat.h
> > > +++ b/include/linux/vmstat.h
> > > @@ -483,6 +483,19 @@ static inline const char *zone_stat_name(enum zone_stat_item item)
> > >  	return vmstat_text[item];
> > >  }
> > >  
> > > +static inline bool zone_appears_fragmented(struct zone *zone)
> > > +{
> > > +	/*
> > > +	 * Simple heuristic: if the number of free pages is more than twice the
> > > +	 * high watermark, this strongly suggests that the zone is heavily
> > > +	 * fragmented when called from a shrinker.
> > > +	 */
> > > +	if (zone_page_state(zone, NR_FREE_PAGES) > high_wmark_pages(zone) * 2)
> > > +		return true;
> > > +
> > > +	return false;
> > > +}
> > > +
> > >  #ifdef CONFIG_NUMA
> > >  static inline const char *numa_stat_name(enum numa_stat_item item)
> > >  {
> > 
> > 
> > Without any usage/users, this is hard to review. I don't understand the heuristic
> > or it's logic as applied to fragmentation either.
> > 
> 
> Sorry—it’s always confusing who to CC on cross-subsystem series. Last
> time this occurred, we agreed to CC everyone listed in the cover letter,
> which I did. Anyway, let me provide the Patchwork links...
> 
> Cover letter: https://patchwork.freedesktop.org/series/165329/
> TTM patch which uses this: https://patchwork.freedesktop.org/patch/720036/?series=165329&rev=1
> Xe side which uses the TTM helper: https://patchwork.freedesktop.org/patch/720031/?series=165329&rev=1
> 

Also if you want grab whole series locally here is what I do when I'm
missed on a Cc:

b4 mbox <msg-id>
mutt -f <msg-id>

So here, with msg-id from cover letter:

b4 mbox 20260423055656.1696379-1-matthew.brost@xxxxxxxxx
mutt -f  ./20260423055656.1696379-1-matthew.brost@xxxxxxxxxxxxx

Matt

> Matt
> 
> > Balbir