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Tue, 31 Mar 2026 02:40:21 -0700 (PDT) Date: Tue, 31 Mar 2026 17:40:13 +0800 From: Kairui Song To: Baolin Wang Cc: kasong@tencent.com, linux-mm@kvack.org, Andrew Morton , Axel Rasmussen , Yuanchu Xie , Wei Xu , Johannes Weiner , David Hildenbrand , Michal Hocko , Qi Zheng , Shakeel Butt , Lorenzo Stoakes , Barry Song , David Stevens , Chen Ridong , Leno Hou , Yafang Shao , Yu Zhao , Zicheng Wang , Kalesh Singh , Suren Baghdasaryan , Chris Li , Vernon Yang , linux-kernel@vger.kernel.org, Qi Zheng Subject: Re: [PATCH v2 12/12] mm/vmscan: unify writeback reclaim statistic and throttling Message-ID: References: <20260329-mglru-reclaim-v2-0-b53a3678513c@tencent.com> <20260329-mglru-reclaim-v2-12-b53a3678513c@tencent.com> <052ae271-509c-42c3-877e-ac8822b314e5@linux.alibaba.com> <522f4898-78c3-453f-8367-29327e29290e@linux.alibaba.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <522f4898-78c3-453f-8367-29327e29290e@linux.alibaba.com> On Tue, Mar 31, 2026 at 05:36:26PM +0800, Baolin Wang wrote: > > > On 3/31/26 5:29 PM, Kairui Song wrote: > > On Tue, Mar 31, 2026 at 05:24:39PM +0800, Baolin Wang wrote: > > > > > > > > > On 3/29/26 3:52 AM, Kairui Song via B4 Relay wrote: > > > > From: Kairui Song > > > > > > > > Currently MGLRU and non-MGLRU handle the reclaim statistic and > > > > writeback handling very differently, especially throttling. > > > > Basically MGLRU just ignored the throttling part. > > > > > > > > Let's just unify this part, use a helper to deduplicate the code > > > > so both setups will share the same behavior. Also remove the > > > > folio_clear_reclaim in isolate_folio which was actively invalidating > > > > the congestion control. PG_reclaim is now handled by shrink_folio_list, > > > > keeping it in isolate_folio is not helpful. > > > > > > > > Test using following reproducer using bash: > > > > > > > > echo "Setup a slow device using dm delay" > > > > dd if=/dev/zero of=/var/tmp/backing bs=1M count=2048 > > > > LOOP=$(losetup --show -f /var/tmp/backing) > > > > mkfs.ext4 -q $LOOP > > > > echo "0 $(blockdev --getsz $LOOP) delay $LOOP 0 0 $LOOP 0 1000" | \ > > > > dmsetup create slow_dev > > > > mkdir -p /mnt/slow && mount /dev/mapper/slow_dev /mnt/slow > > > > > > > > echo "Start writeback pressure" > > > > sync && echo 3 > /proc/sys/vm/drop_caches > > > > mkdir /sys/fs/cgroup/test_wb > > > > echo 128M > /sys/fs/cgroup/test_wb/memory.max > > > > (echo $BASHPID > /sys/fs/cgroup/test_wb/cgroup.procs && \ > > > > dd if=/dev/zero of=/mnt/slow/testfile bs=1M count=192) > > > > > > > > echo "Clean up" > > > > echo "0 $(blockdev --getsz $LOOP) error" | dmsetup load slow_dev > > > > dmsetup resume slow_dev > > > > umount -l /mnt/slow && sync > > > > dmsetup remove slow_dev > > > > > > > > Before this commit, `dd` will get OOM killed immediately if > > > > MGLRU is enabled. Classic LRU is fine. > > > > > > > > After this commit, congestion control is now effective and no more > > > > spin on LRU or premature OOM. > > > > > > > > Stress test on other workloads also looking good. > > > > > > > > Suggested-by: Chen Ridong > > > > Signed-off-by: Kairui Song > > > > --- > > > > mm/vmscan.c | 93 +++++++++++++++++++++++++++---------------------------------- > > > > 1 file changed, 41 insertions(+), 52 deletions(-) > > > > > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c > > > > index 1783da54ada1..83c8fdf8fdc4 100644 > > > > --- a/mm/vmscan.c > > > > +++ b/mm/vmscan.c > > > > @@ -1942,6 +1942,44 @@ static int current_may_throttle(void) > > > > return !(current->flags & PF_LOCAL_THROTTLE); > > > > } > > > > +static void handle_reclaim_writeback(unsigned long nr_taken, > > > > + struct pglist_data *pgdat, > > > > + struct scan_control *sc, > > > > + struct reclaim_stat *stat) > > > > +{ > > > > + /* > > > > + * If dirty folios are scanned that are not queued for IO, it > > > > + * implies that flushers are not doing their job. This can > > > > + * happen when memory pressure pushes dirty folios to the end of > > > > + * the LRU before the dirty limits are breached and the dirty > > > > + * data has expired. It can also happen when the proportion of > > > > + * dirty folios grows not through writes but through memory > > > > + * pressure reclaiming all the clean cache. And in some cases, > > > > + * the flushers simply cannot keep up with the allocation > > > > + * rate. Nudge the flusher threads in case they are asleep. > > > > + */ > > > > + if (stat->nr_unqueued_dirty == nr_taken && nr_taken) { > > > > + wakeup_flusher_threads(WB_REASON_VMSCAN); > > > > + /* > > > > + * For cgroupv1 dirty throttling is achieved by waking up > > > > + * the kernel flusher here and later waiting on folios > > > > + * which are in writeback to finish (see shrink_folio_list()). > > > > + * > > > > + * Flusher may not be able to issue writeback quickly > > > > + * enough for cgroupv1 writeback throttling to work > > > > + * on a large system. > > > > + */ > > > > + if (!writeback_throttling_sane(sc)) > > > > + reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK); > > > > + } > > > > + > > > > + sc->nr.dirty += stat->nr_dirty; > > > > + sc->nr.congested += stat->nr_congested; > > > > + sc->nr.writeback += stat->nr_writeback; > > > > + sc->nr.immediate += stat->nr_immediate; > > > > + sc->nr.taken += nr_taken; > > > > +} > > > > + > > > > /* > > > > * shrink_inactive_list() is a helper for shrink_node(). It returns the number > > > > * of reclaimed pages > > > > @@ -2005,39 +2043,7 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan, > > > > lruvec_lock_irq(lruvec); > > > > lru_note_cost_unlock_irq(lruvec, file, stat.nr_pageout, > > > > nr_scanned - nr_reclaimed); > > > > - > > > > - /* > > > > - * If dirty folios are scanned that are not queued for IO, it > > > > - * implies that flushers are not doing their job. This can > > > > - * happen when memory pressure pushes dirty folios to the end of > > > > - * the LRU before the dirty limits are breached and the dirty > > > > - * data has expired. It can also happen when the proportion of > > > > - * dirty folios grows not through writes but through memory > > > > - * pressure reclaiming all the clean cache. And in some cases, > > > > - * the flushers simply cannot keep up with the allocation > > > > - * rate. Nudge the flusher threads in case they are asleep. > > > > - */ > > > > - if (stat.nr_unqueued_dirty == nr_taken) { > > > > - wakeup_flusher_threads(WB_REASON_VMSCAN); > > > > - /* > > > > - * For cgroupv1 dirty throttling is achieved by waking up > > > > - * the kernel flusher here and later waiting on folios > > > > - * which are in writeback to finish (see shrink_folio_list()). > > > > - * > > > > - * Flusher may not be able to issue writeback quickly > > > > - * enough for cgroupv1 writeback throttling to work > > > > - * on a large system. > > > > - */ > > > > - if (!writeback_throttling_sane(sc)) > > > > - reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK); > > > > - } > > > > - > > > > - sc->nr.dirty += stat.nr_dirty; > > > > - sc->nr.congested += stat.nr_congested; > > > > - sc->nr.writeback += stat.nr_writeback; > > > > - sc->nr.immediate += stat.nr_immediate; > > > > - sc->nr.taken += nr_taken; > > > > - > > > > + handle_reclaim_writeback(nr_taken, pgdat, sc, &stat); > > > > trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id, > > > > nr_scanned, nr_reclaimed, &stat, sc->priority, file); > > > > return nr_reclaimed; > > > > @@ -4651,9 +4657,6 @@ static bool isolate_folio(struct lruvec *lruvec, struct folio *folio, struct sca > > > > if (!folio_test_referenced(folio)) > > > > set_mask_bits(&folio->flags.f, LRU_REFS_MASK, 0); > > > > - /* for shrink_folio_list() */ > > > > - folio_clear_reclaim(folio); > > > > > > IMO, Moving this change into patch 8 would make more sense. Otherwise LGTM. > > > > Thanks for the review! I made it a separate patch so we can better > > identify which part had the performance gain, and patch 8 can keep > > the review by. Patch 8 is still good without this, a few counters > > are updated with no user, kind of wasted but that's harmless. > > I’m not referring to all the above changes. What I mean is that the > 'folio_clear_reclaim' removal should belong to patch 8. Since > shrink_folio_list() in patch 8 will handle the writeback logic, > folio_clear_reclaim() should also be removed in the same patch. Ah, that's a very good point then. Can move it in V3, thanks!