Re: [PATCH linux-next] mm/page_alloc: avoid overcounting bulk allocin watermark check

[shengminghu512]

> On Thu, 29 Jan 2026 22:38:14 +0800 "shengminghu512" <shengminghu512@xxxxxx> wrote:
> 
> > From: Shengming Hu <hu.shengming@xxxxxxxxxx>
> > 
> > alloc_pages_bulk_noprof() only fills NULL slots and already tracks how many
> > entries are pre-populated via nr_populated.
> > 
> > The fast watermark check was adding nr_pages unconditionally, which can
> > overestimate the demand. Use (nr_pages - nr_populated) instead, as an 
> > upper bound on the remaining pages this call can still allocate without
> > scanning the whole array.
> 
> Thanks.
> 
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -5130,7 +5130,7 @@ unsigned long alloc_pages_bulk_noprof(gfp_t gfp, int preferred_nid,
> >  
> >   cond_accept_memory(zone, 0, alloc_flags);
> >  retry_this_zone:
> > - mark = wmark_pages(zone, alloc_flags & ALLOC_WMARK_MASK) + nr_pages;
> > + mark = wmark_pages(zone, alloc_flags & ALLOC_WMARK_MASK) + nr_pages - nr_populated;
> >   if (zone_watermark_fast(zone, 0,  mark,
> >   zonelist_zone_idx(ac.preferred_zoneref),
> >   alloc_flags, gfp)) {
> 
> So that little optimization hasn't been working for four years?

Yeah, looks like it’s been conservative for a long time :)

It didn’t break correctness, but it likely made the fast watermark check
less effective by overestimating demand (counting already-populated entries
again), so we’d drop out of `zone_watermark_fast()` earlier and hit the
slow path more often.

--
With Best Regards,
Shengming