Re: [PATCH v4] mm/page_alloc: avoid direct compaction for costly __GFP_NORETRY allocations

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Sep 18, 2026 at 09:05:40AM +0200, Vlastimil Babka (SUSE) wrote:
> On 9/16/26 17:58, Johannes Weiner wrote:
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 12fac9084c48..c79cc7aa4dff 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -3246,7 +3246,8 @@ struct page *rmqueue_buddy(struct zone *preferred_zone, struct zone *zone,
> >  			 * reserves as failing now is worse than failing a
> >  			 * high-order atomic allocation in the future.
> >  			 */
> > -			if (!page && (alloc_flags & (ALLOC_OOM|ALLOC_NON_BLOCK)))
> > +			if (!page && ((alloc_flags & ALLOC_OOM) ||
> > +				      ((alloc_flags & ALLOC_MASK_ATOMIC) == ALLOC_MASK_ATOMIC)))
> >  				page = __rmqueue_smallest(zone, order, MIGRATE_HIGHATOMIC);
> >  
> >  			if (!page) {
> > diff --git a/mm/page_alloc.h b/mm/page_alloc.h
> > index b9259deddb59..11714ddca254 100644
> > --- a/mm/page_alloc.h
> > +++ b/mm/page_alloc.h
> > @@ -60,6 +60,9 @@
> >  /* Flags that allow allocations below the min watermark. */
> >  #define ALLOC_RESERVES (ALLOC_NON_BLOCK|ALLOC_MIN_RESERVE|ALLOC_HIGHATOMIC|ALLOC_OOM)
> >  
> > +/* Flag combination from GFP_ATOMIC */
> > +#define ALLOC_MASK_ATOMIC (ALLOC_NON_BLOCK|ALLOC_MIN_RESERVE)
> > +
> >  /*
> >   * Structure for holding the mostly immutable allocation parameters passed
> >   * between functions involved in allocations, including the alloc_pages*
> 
> Should work.
> 
> > 
> >> Unless I'm mistaken about that MIGRATE_HIGHATOMIC part, it seems all sashiko
> >> concerns can be dismissed and then indeed v4 is the better version.
> > 
> > It looks like a real issue to me, but one that already exists
> > independent of Salvatore's change.
> 
> But Salvatore's change (v4, not v5 IIUC) will make it worse because
> __GFP_DIRECT_RECLAIM + __GFP_NORETRY costly order opportunistic attempts
> with fallback will start eating the highatomic reserves too?
>
> In that case the fix for this should probably be part of the same PR to
> Linus and be also stable with Fixes: 281dd25c1a01

Yes, agreed on both.

Sorry for the delay - while poking around I found another oddity in
how the reserves are accessed.

I'll reply with two patches that I think we want to go in as fixes,
whether Salvatore's patch goes ahead or not.