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

[Matthew Brost]

On Thu, Apr 23, 2026 at 01:27:11PM +0200, Thomas Hellström wrote:
> 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

If the order were included in shrink_control, there is about a 95%
certain that this change would allow TTM / Xe to break the problematic
kswapd feedback loop. This may also better express the intent of the
problem we are trying to fix here.

For reference, the cover letter [1] details the problem.

Any guidance from the core MM folks would be appreciated—would adding
the order to shrink_control be an acceptable solution?

Matt

[1] https://patchwork.freedesktop.org/series/165330/

> 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
> 
> 
> 
>