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

[Kairui Song <ryncsn@gmail.com>]

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@tencent.com>
> > 
> > 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@google.com>
> > Link: https://lore.kernel.org/linux-mm/20241026115714.1437435-1-jingxiangzeng.cas@gmail.com/ [1]
> > Signed-off-by: Kairui Song <kasong@tencent.com>
> > ---
> >   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?

> 
> As a result, during the next eviction, these dirty folios won't be scanned
> again (because they are in the second youngest generation). Wouldn't this
> lead to a situation where the flusher cannot be woken up in time, making OOM
> more likely?

No? Flusher is already waken up by the time they are seen for the
first time. If we see these folios again very soon, the LRU is
congested, one following patch handles the congested case too by
throttling (which was completely missing previously). And now we
are actually a bit more proactive about waking up the flusher,
since the wakeup hook is moved inside the loop instead of after
the whole loop is finished.

These two behavior change above is basically just unifying MGLRU to do
what the classical LRU has been doing for years, and result looks
really good.

The global congestion handling is still missing after this series
though. Have to fix that later I guess...