Re: [PATCH] mm/mglru: skip oom killing for high order allocs in kswapd

[Kairui Song]

On Wed, Oct 28, 2026 at 9:56 PM Kairui Song <ryncsn@xxxxxxxxx> wrote:
>
> On Tue, Oct 27, 2026 at 10:21 AM Bingfang Guo via B4 Relay
> <devnull+bingfangguo.tencent.com@xxxxxxxxxx> wrote:
> >
> > From: Bingfang Guo <bingfangguo@xxxxxxxxxxx>
> >
> > We observed that when MGLRU min_ttl is set and the system is highly
> > fragmented, high order allocations (e.g. order == 9) will fail and wake
> > up kswapd. It can not make any progress, while pushing generations of
> > memcgs so that they all end up younger than min_ttl_ms, repeatedly
> > triggering the OOM killer in lru_gen_age_node(). Then the whole process
> > repeats until all processes are killed and the kernel panics.
> >
> > Fix this by skipping OOM killer invocations in lru_gen_age_node() for
> > sc->order > PAGE_ALLOC_COSTLY_ORDER, which is also what the normal
> > allocation path will do, to fail high order allocations without OOM
> > killing.
> >
> > ==== Testing ====
> >
> > Tested on a VM with 6 cores and 6GiB of memory, HZ=1000, MGLRU enabled,
> > min_ttl_ms set to 1000, and a workload that fragments the memory then
> > allocates 2M pages.
> >
> > Without the patch, kswapd kills all processes and panics the system even
> > with plenty of memory available. With the patch, allocations fail but
> > the system remains stable.
> >
> > Fixes: 30d77b7eef019 ("mm/mglru: fix ineffective protection calculation")
> > Signed-off-by: Bingfang Guo <bingfangguo@xxxxxxxxxxx>
> > ---
> >  mm/vmscan.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
>
> Thanks! Really nice catch.
>
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 9959ca158ce83..b32777468ead8 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -4407,6 +4407,12 @@ static void lru_gen_age_node(struct pglist_data *pgdat, struct scan_control *sc)
> >                         reclaimable = lruvec_is_reclaimable(lruvec, sc, min_ttl);
> >         } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)));
> >
> > +       /*
> > +        * OOM killing won't help high order allocations. Skip and let them fail.
>
> A nit pick, OOM definately helps for high order allocations :-D, maybe
> something like, the OOM kill is to prevent thrashing whtn all memcgs
> are younger than min_ttl (and maybe merge with the comment & check
> below?), so OOM for fragmentation is definitely unwanted.
>
> > +        */
> > +       if (sc->order > PAGE_ALLOC_COSTLY_ORDER)
> > +               return;
> > +
> >         /*
> >          * The main goal is to OOM kill if every generation from all memcgs is
> >          * younger than min_ttl. However, another possibility is all memcgs are
>
> This patch is good to me on its own with a nit fix above.
>
> And checking commit 30d77b7eef019, you might need a Fixes: or maybe
> even bring the check back which was removed in that commit?

Oh I noticed it's already there, please ignore this one :P