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

[Thomas Hellström]

On Thu, 2026-04-23 at 12:27 +0200, David Hildenbrand (Arm) wrote:
> On 4/23/26 07:56, 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)
> > +{
> 
> "zone_likely_fragmented" or "zone_maybe_fragmented" might be clearer,
> depending
> on the actual semantics.
> 
> > +	/*
> > +	 * 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.
> > +	 */
> 
> I'll cc some more people. But the "when called from a shrinker" bit
> is
> concerning. Are there additional semantics that should be expressed
> in the
> function name, for example?
> 
> Something that implies that this function only gives you a reasonable
> answer in
> a certain context.

I think that test would not be relevant for cgroup-aware shrinking.

What about trying to pass something in the struct shrink_control? Like
if we pass the struct scan_control's order field also in struct
shrink_control, really expensive shrinkers could duck reclaim attempts
from higher-order allocations that may fail anyway:

      if (sc->order > PAGE_ALLOC_COSTLY_ORDER &&
           (sc->gfp_mask & (__GFP_NORETRY | __GFP_RETRY_MAYFAIL)) &&
           !(sc->gfp_mask & __GFP_NOFAIL))
           return SHRINK_STOP;

Possibly exposed as an inline helper in the shrinker interface?

/Thomas