Re: [PATCH v2 08/12] mm/mglru: simplify and improve dirty writeback handling

[Barry Song]

On Tue, Mar 31, 2026 at 5:18 PM Kairui Song <ryncsn@xxxxxxxxx> wrote:
>
> On Tue, Mar 31, 2026 at 04:42:59PM +0800, Baolin Wang wrote:
> >
> >
> > On 3/29/26 3:52 AM, Kairui Song via B4 Relay wrote:
> > > From: Kairui Song <kasong@xxxxxxxxxxx>
> > >
> > > The current handling of dirty writeback folios is not working well for
> > > file page heavy workloads: Dirty folios are protected and move to next
> > > gen upon isolation of getting throttled or reactivation upon pageout
> > > (shrink_folio_list).
> > >
> > > This might help to reduce the LRU lock contention slightly, but as a
> > > result, the ping-pong effect of folios between head and tail of last two
> > > gens is serious as the shrinker will run into protected dirty writeback
> > > folios more frequently compared to activation. The dirty flush wakeup
> > > condition is also much more passive compared to active/inactive LRU.
> > > Active / inactve LRU wakes the flusher if one batch of folios passed to
> > > shrink_folio_list is unevictable due to under writeback, but MGLRU
> > > instead has to check this after the whole reclaim loop is done, and then
> > > count the isolation protection number compared to the total reclaim
> > > number.
> > >
> > > And we previously saw OOM problems with it, too, which were fixed but
> > > still not perfect [1].
> > >
> > > So instead, just drop the special handling for dirty writeback, just
> > > re-activate it like active / inactive LRU. And also move the dirty flush
> > > wake up check right after shrink_folio_list. This should improve both
> > > throttling and performance.
> > >
> > > Test with YCSB workloadb showed a major performance improvement:
> > >
> > > Before this series:
> > > Throughput(ops/sec): 61642.78008938203
> > > AverageLatency(us):  507.11127774145166
> > > pgpgin 158190589
> > > pgpgout 5880616
> > > workingset_refault 7262988
> > >
> > > After this commit:
> > > Throughput(ops/sec): 80216.04855744806  (+30.1%, higher is better)
> > > AverageLatency(us):  388.17633477268913 (-23.5%, lower is better)
> > > pgpgin 101871227                        (-35.6%, lower is better)
> > > pgpgout 5770028
> > > workingset_refault 3418186              (-52.9%, lower is better)
> > >
> > > The refault rate is ~50% lower, and throughput is ~30% higher, which
> > > is a huge gain. We also observed significant performance gain for
> > > other real-world workloads.
> > >
> > > We were concerned that the dirty flush could cause more wear for SSD:
> > > that should not be the problem here, since the wakeup condition is when
> > > the dirty folios have been pushed to the tail of LRU, which indicates
> > > that memory pressure is so high that writeback is blocking the workload
> > > already.
> > >
> > > Reviewed-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
> > > Link: https://lore.kernel.org/linux-mm/20241026115714.1437435-1-jingxiangzeng.cas@xxxxxxxxx/ [1]
> > > Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> > > ---
> > >   mm/vmscan.c | 57 ++++++++++++++++-----------------------------------------
> > >   1 file changed, 16 insertions(+), 41 deletions(-)
> > >
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index 8de5c8d5849e..17b5318fad39 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -4583,7 +4583,6 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_c
> > >                    int tier_idx)
> > >   {
> > >     bool success;
> > > -   bool dirty, writeback;
> > >     int gen = folio_lru_gen(folio);
> > >     int type = folio_is_file_lru(folio);
> > >     int zone = folio_zonenum(folio);
> > > @@ -4633,21 +4632,6 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_c
> > >             return true;
> > >     }
> > > -   dirty = folio_test_dirty(folio);
> > > -   writeback = folio_test_writeback(folio);
> > > -   if (type == LRU_GEN_FILE && dirty) {
> > > -           sc->nr.file_taken += delta;
> > > -           if (!writeback)
> > > -                   sc->nr.unqueued_dirty += delta;
> > > -   }
> > > -
> > > -   /* waiting for writeback */
> > > -   if (writeback || (type == LRU_GEN_FILE && dirty)) {
> > > -           gen = folio_inc_gen(lruvec, folio, true);
> > > -           list_move(&folio->lru, &lrugen->folios[gen][type][zone]);
> > > -           return true;
> > > -   }
> >
> > I'm a bit concerned about the handling of dirty folios.
> >
> > In the original logic, if we encounter a dirty folio, we increment its
> > generation counter by 1 and move it to the *second oldest generation*.
> >
> > However, with your patch, shrink_folio_list() will activate the dirty folio
> > by calling folio_set_active(). Then, evict_folios() -> move_folios_to_lru()
> > will put the dirty folio back into the MGLRU list.
> >
> > But because the folio_test_active() is true for this dirty folio, the dirty
> > folio will now be placed into the *second youngest generation* (see
> > lru_gen_folio_seq()).
>
> Yeah, and that's exactly what we want. Or else, these folios will
> stay at oldest gen, following scan will keep seeing them and hence
> keep bouncing these folios again and again to a younger gen since
> they are not reclaimable.
>
> The writeback callback (folio_rotate_reclaimable) will move them
> back to tail once they are actually reclaimable. So we are not
> losing any ability to reclaim them. Am I missing anything?
>

This makes sense to me. As long as folio_rotate_reclaimable()
exists, we can move those folios back to the tail once they are
clean and ready for reclaim.

This reminds me of Ridong's patch, which tried to emulate MGLRU's
behavior by 'rotating' folios whose IO completed during isolate,
and thus missed folio_rotate_reclaimable() in the active/inactive
LRUs[1]. Not sure if that patch has managed to land since v7.

                /* retry folios that may have missed
folio_rotate_reclaimable() */
                if (!skip_retry && !folio_test_active(folio) &&
!folio_mapped(folio) &&
                    !folio_test_dirty(folio) && !folio_test_writeback(folio)) {
                        list_move(&folio->lru, &clean);
                        continue;
                }

[1] https://lore.kernel.org/linux-mm/20250111091504.1363075-1-chenridong@xxxxxxxxxxxxxxx/

Best Regards
Barry