Re: [PATCH v4 3/3] mm/vmscan: don't demote if there is not enough free memory in the lower memory tier

[Akinobu Mita <akinobu.mita@gmail.com>]

2026年1月28日(水) 6:21 Gregory Price <gourry@gourry.net>:
>
> On Mon, Jan 26, 2026 at 10:57:11AM +0900, Akinobu Mita wrote:
> > >
> > > Doesn't this suggest what I mentioned earlier?  If you don't demote when
> > > the target node is full, then you're removing a memory pressure signal
> > > from the lower node and reclaim won't ever clean up the lower node to
> > > make room for future demotions.
> >
> > Thank you for your analysis.
> > Now I finally understand the concerns (though I'll need to learn more
> > to find a solution...)
> >
>
> Apologies - sorry for the multiple threads, i accidentally replied on v3
>
> It's taken me a while to detangle this, but what looks  like what might
> be happening is demote_folios is actually stealing all the potential
> candidates for swap for leaving reclaim with no forward progress and no
> OOM signal.
>
> 1) demotion is already not a reclaim signal, so forgive my prior
>    comments, i missed the masking of ~__GFP_RECLAIM
>
> 2) it appears we spend most of the time building the demotion list, but
>    then just abandon the list without having made progress later when
>    the demotion allocation target fails (w/ __THISNODE you don't get
>    OOM on allocation failure, we just continue)
>
> 3) i don't see hugetlb pages causing the GFP_RECLAIM override bug being
>    an issue in reclaim, because the page->lru is used for something else
>    in hugetlb pages (i.e. we shouldn't see hugetlb pages here)
>
> 4) skipping the entire demotion pass will shunt all this pressure to
>    swap instead (do_demote_pass = false -> so we swap instead).
>
>
> The risk here is that the OOM situation is temporary and some amount of
> memory from toptier gets shunting to swap while kswapd on other tiers
> makes progress.  This is effectively LRU inversion.
>
> Why swappiness affects behavior is likely because it changes how
> aggressively your lower-tier gets reclaimed, and therefore reduces the
> upper tier demotion failures until swap is already pressured.
>
> I'm not sure there's a best-option here, we may need additional input to
> determine what the least-worst option is.  Causing LRU inversion when
> all the nodes are pressured but swap is available is not preferable.

Would it be better if can_demote() returned false after checking that
there is no free swap space at all and that there is not enough free space
on the demote target node or its lower nodes?

can_demote()
{
        ...
        /* If demotion node isn't in the cgroup's mems_allowed, fall back */
        if (mem_cgroup_node_allowed(memcg, demotion_nid)) {
                if (get_nr_swap_pages() > 0)
                        return true;
                do {
                        int z;
                        struct zone *zone;
                        struct pglist_data *pgdat = NODE_DATA(demotion_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))
                                        return true;
                        }
                        demotion_nid = next_demotion_node(demotion_nid);
                } while (demotion_nid != NUMA_NO_NODE);
        }
        return false;
}