Message-ID: <aXndXPMFK2fhLA4p@tiehlicka>
Date: Wed, 28 Jan 2026 10:56:44 +0100
From: Michal Hocko <mhocko@...e.com>
To: Gregory Price <gourry@...rry.net>
Cc: Akinobu Mita <akinobu.mita@...il.com>, linux-cxl@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	akpm@...ux-foundation.org, axelrasmussen@...gle.com,
	yuanchu@...gle.com, weixugc@...gle.com, hannes@...xchg.org,
	david@...nel.org, zhengqi.arch@...edance.com,
	shakeel.butt@...ux.dev, lorenzo.stoakes@...cle.com,
	Liam.Howlett@...cle.com, vbabka@...e.cz, rppt@...nel.org,
	surenb@...gle.com, bingjiao@...gle.com
Subject: Re: [PATCH v3 3/3] mm/vmscan: don't demote if there is not enough
 free memory in the lower memory tier

On Tue 27-01-26 15:24:36, Gregory Price wrote:
> On Sat, Jan 10, 2026 at 10:55:02PM +0900, Akinobu Mita wrote:
> > 2026年1月10日(土) 1:08 Gregory Price <gourry@...rry.net>:
> > >
> > > > +     for_each_node_mask(nid, allowed_mask) {
> > > > +             int z;
> > > > +             struct zone *zone;
> > > > +             struct pglist_data *pgdat = NODE_DATA(nid);
> > > > +
> > > > +             for_each_managed_zone_pgdat(zone, pgdat, z, MAX_NR_ZONES - 1) {
> > > > +                     if (zone_watermark_ok(zone, 0, min_wmark_pages(zone),
> > > > +                                             ZONE_MOVABLE, 0))
> > >
> > > Why does this only check zone movable?
> > 
> > Here, zone_watermark_ok() checks the free memory for all zones from 0 to
> > MAX_NR_ZONES - 1.
> > There is no strong reason to pass ZONE_MOVABLE as the highest_zoneidx
> > argument every time zone_watermark_ok() is called; I can change it if an
> > appropriate value is found.
> > In v1, highest_zoneidx was "sc ? sc->reclaim_idx : MAX_NR_ZONES - 1"
> > 
> > > Also, would this also limit pressure-signal to invoke reclaim when
> > > there is still swap space available?  Should demotion not be a pressure
> > > source for triggering harder reclaim?
> > 
> > Since can_reclaim_anon_pages() checks whether there is free space on the swap
> > device before checking with can_demote(), I think the negative impact of this
> > change will be small. However, since I have not been able to confirm the
> > behavior when a swap device is available, I would like to correctly understand
> > the impact.
> 
> Something else is going on here
> 
> See demote_folio_list and alloc_demote_folio
> 
> static unsigned int demote_folio_list(struct list_head *demote_folios,
>                                       struct pglist_data *pgdat,
>                                       struct mem_cgroup *memcg)
> {
>         struct migration_target_control mtc = {
>                  */
>                 .gfp_mask = (GFP_HIGHUSER_MOVABLE & ~__GFP_RECLAIM) |
>                         __GFP_NOMEMALLOC | GFP_NOWAIT,
>         };
> }
> 
> static struct folio *alloc_demote_folio(struct folio *src,
>                 unsigned long private)
> {
> 	/* Only attempt to demote to the preferred node */
>         mtc->nmask = NULL;
>         mtc->gfp_mask |= __GFP_THISNODE;
>         dst = alloc_migration_target(src, (unsigned long)mtc);
>         if (dst)
>                 return dst;
> 
> 	/* Now attempt to demote to any node in the lower tier */
>         mtc->gfp_mask &= ~__GFP_THISNODE;
>         mtc->nmask = allowed_mask;
>         return alloc_migration_target(src, (unsigned long)mtc);
> }
> 
> 
> /*
> * %__GFP_RECLAIM is shorthand to allow/forbid both direct and kswapd reclaim.
> */
> 
> 
> You basically shouldn't be hitting any reclaim behavior at all, and if

This will trigger kswapd so there will be background reclaim demoting
from those lower tiers.

> the target nodes are actually under various watermarks, you should be
> getting allocation failures and quick-outs from the demotion logic.
> 
> i.e. you should be seeing OOM happen
> 
> When I dug in far enough I found this:
> 
> static struct folio *alloc_demote_folio(struct folio *src,
>                 unsigned long private)
> {
> ...
>         dst = alloc_migration_target(src, (unsigned long)mtc);
> }
> 
> struct folio *alloc_migration_target(struct folio *src, unsigned long private)
> {
>         
> ...
>         if (folio_test_hugetlb(src)) {
>                 struct hstate *h = folio_hstate(src);
> 
>                 gfp_mask = htlb_modify_alloc_mask(h, gfp_mask);
>                 return alloc_hugetlb_folio_nodemask(h, nid, ...)
> 	}
> }
> 
> static inline gfp_t htlb_modify_alloc_mask(struct hstate *h, gfp_t gfp_mask)
> {
>         gfp_t modified_mask = htlb_alloc_mask(h);
> 
>         /* Some callers might want to enforce node */
>         modified_mask |= (gfp_mask & __GFP_THISNODE);
> 
>         modified_mask |= (gfp_mask & __GFP_NOWARN);
> 
>         return modified_mask;
> }
> 
> /* Movability of hugepages depends on migration support. */
> static inline gfp_t htlb_alloc_mask(struct hstate *h)
> {
>         gfp_t gfp = __GFP_COMP | __GFP_NOWARN;
> 
>         gfp |= hugepage_movable_supported(h) ? GFP_HIGHUSER_MOVABLE : GFP_HIGHUSER;
> 
>         return gfp;
> }
> 
> #define GFP_USER        (__GFP_RECLAIM | __GFP_IO | __GFP_FS | __GFP_HARDWALL)
> #define GFP_HIGHUSER    (GFP_USER | __GFP_HIGHMEM)
> #define GFP_HIGHUSER_MOVABLE    (GFP_HIGHUSER | __GFP_MOVABLE | __GFP_SKIP_KASAN)
> 
> 
> If we try to move a hugepage, we start including __GFP_RECLAIM again -
> regardless of whether HIGHUSER_MOVABLE or HIGHUSER is used.
> 
> 
> Any chance you are using hugetlb on this system?  This looks like a
> clear bug, but it may not be what you're experiencing.

Hugetlb pages are not sitting on LRU lists so they are not participating
in the demotion.

Or maybe I missed your point.
-- 
Michal Hocko
SUSE Labs

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