Re: [PATCHv3] mm: remove '!root_reclaim' checking in should_abort_scan()

[Zhaoyang Huang]

On Fri, Apr 3, 2026 at 6:59 AM Barry Song <baohua@xxxxxxxxxx> wrote:
>
> On Wed, Mar 18, 2026 at 9:18 AM zhaoyang.huang
> <zhaoyang.huang@xxxxxxxxxx> wrote:
> >
> > From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> >
> > Android systems usually use memory.reclaim interface to implement user
> > space memory management which expects that the requested reclaim target
> > and actually reclaimed amount memory are not diverging by too much. With
> > the current MGRLU implementation there is, however, no bail out when the
> > reclaim target is reached and this could lead to an excessive reclaim
> > that scales with the reclaim hierarchy size.For example, we can get a
> > nr_reclaimed=394/nr_to_reclaim=32 proactive reclaim under a common 1-N
> > cgroup hierarchy.
> > This defect arised from the goal of keeping fairness among memcgs that
> > is, for try_to_free_mem_cgroup_pages -> shrink_node_memcgs ->
> > shrink_lruvec -> lru_gen_shrink_lruvec -> try_to_shrink_lruvec, the
> > !root_reclaim(sc) check was there for reclaim fairness, which was
> > necessary before commit 'b82b530740b9' ("mm: vmscan: restore
> > incremental cgroup iteration") because the fairness depended on
> > attempted proportional reclaim from every memcg under the target
> > memcg. However after commit 'b82b530740b9' there is no longer a need
> > to visit every memcg to ensure fairness. Let's have try_to_shrink_lruvec
> > bail out when the nr_reclaimed achieved.
>
> I think we need some clarification here. Does the code
>
> nr_to_scan = apply_proportional_protection(memcg, sc, nr_to_scan);
>
> still serve a purpose, or has it become less useful after your patch?
proportional protection is still useful for calculating the nr_to_scan
when 'memory.min/low' is configured in the hierarchy.

>
> >
> > Suggested-by: T.J.Mercier <tjmercier@xxxxxxxxxx>
> > Reviewed-by: T.J.Mercier <tjmercier@xxxxxxxxxx>
> > Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> > ---
> > Patchv2,v3: update commit message
> > ---
> > ---
> >  mm/vmscan.c | 4 ----
> >  1 file changed, 4 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 0fc9373e8251..10f1e7d716ca 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -4839,10 +4839,6 @@ static bool should_abort_scan(struct lruvec *lruvec, struct scan_control *sc)
> >         int i;
> >         enum zone_watermarks mark;
> >
> > -       /* don't abort memcg reclaim to ensure fairness */
> > -       if (!root_reclaim(sc))
> > -               return false;
> > -
> >         if (sc->nr_reclaimed >= max(sc->nr_to_reclaim, compact_gap(sc->order)))
> >                 return true;
> >
>
> Thanks
> Barry