Re: [PATCH 3/4] mm: Fix demotion gfp by clearing GFP_RECLAIM after setting GFP_TRANSHUGE

[Gregory Price <gourry@gourry.net>]

On Wed, Mar 11, 2026 at 01:54:50PM -0400, Johannes Weiner wrote:
> On Wed, Mar 11, 2026 at 12:02:42PM +0100, Alexandre Ghiti wrote:
> > GFP_TRANSHUGE sets __GFP_DIRECT_RECLAIM so we must clear GFP_RECLAIM
> > after, not before.
> > 
> > Reported-by: Bing Jiao <bingjiao@google.com>
> > Closes: https://lore.kernel.org/linux-mm/aXlKOxGGI9zne8sl@google.com/
> > Fixes: 9933a0c8a539 ("mm/migrate: clear __GFP_RECLAIM to make the migration callback consistent with regular THP allocations")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
> > ---
> >  mm/migrate.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 2c3d489ecf51..ee533a4d38db 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -2190,12 +2190,12 @@ struct folio *alloc_migration_target(struct folio *src, unsigned long private)
> >  	}
> >  
> >  	if (folio_test_large(src)) {
> > +		gfp_mask |= GFP_TRANSHUGE;
> >  		/*
> >  		 * clear __GFP_RECLAIM to make the migration callback
> >  		 * consistent with regular THP allocations.
> >  		 */
> >  		gfp_mask &= ~__GFP_RECLAIM;
> > -		gfp_mask |= GFP_TRANSHUGE;
> 
> I don't think this is right.
> 
> The Fixes: did it this way to disable kswapd for THP allocations,
> while still allowing the customary direct reclaim. Maybe a better
> comment would have been: /* GFP_TRANSHUGE has its own reclaim policy */
> 

The bigger issue how many times we see this particular flag getting
masked and apparently added back in at multiple layers. We saw two
or three paths (some unreachable) that can twiddle RECLAIM flags in
the stack for demotion (which is in reclaim already, so do the flags
matter?).

It makes it difficult to reason about what the GFP flags actually
are at any given point.

But yeah I wasn't sure to make of this code, it could be as you
suggested just a bad comment.

~Gregory