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Howlett" , Vlastimil Babka , Yu Zhao , Zi Yan , Qi Zheng , cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, Kairui Song Subject: Re: [PATCH v2 1/6] mm/memcontrol: make lru_zone_size atomic and simplify sanity check Message-ID: References: <20260824-mglru-flags-cleanup-v2-0-0104132114ae@tencent.com> <20260824-mglru-flags-cleanup-v2-1-0104132114ae@tencent.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260824-mglru-flags-cleanup-v2-1-0104132114ae@tencent.com> X-Rspam-User: X-Stat-Signature: az3w1dmybefpc68usf9dx4fquizw33tc X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 7182840009 X-HE-Tag: 1787622504-237122 X-HE-Meta: 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 0k+4HE+Z 6035CjpugEheUi01ia6Pw42Y4AdGLWYSE13qSWk+FhfH97qsrtkMlEwJklyAIKwwMLUN8ZPKAcMBYqUUSHMn69FMXtTcFYHLTR1xVHwoPjIIS2h4htujK+epjXtaxBbiWMwN9M1eSQ3+49vvjGQmseo6ECs0pv4W+in5AIALDwDGuD7q0ph5dnDymKZIyivJwWqf2GBA7IOZ7Hu9jaW1aRrbZX+9CoSCRZZOcS9t7eyi/MK6sZy87icu+L5JmnzVGiGjHEVJ4nf7zvv5tJKq7/JiPK7FQMXAh9m07VCAc3J4LLfNr3Q5VWzxEfQND6jhriYpDHP1nP6fBcK9mENI+gfO4Edq76hAR/KiLTCZepMzXB41LCE6a6AdSVYw2/YDR0w6v/9SiTqUZ9XtVcB0WaDmgxOx1a/2vJ09rZqgReNZDq92pXj4VdVcRE3mvCjHmf3YNDlIF7bf4ccdms7xoeljFBwEa0Q+mQFKM Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 08/24/26 at 11:26pm, Kairui Song via B4 Relay wrote: > From: Kairui Song > > commit ca707239e8a7 ("mm: update_lru_size warn and reset bad lru_size") > introduced a sanity check to catch memcg counter underflow, which was > more of a workaround for another bug: lru_zone_size is unsigned, so > underflow wraps it around and returns an enormously large number, then > the memcg shrinker loops almost forever as the calculated number of > folios to shrink is huge. That commit also checked if a zero value > matches the empty LRU list, so we have to hold the LRU lock, and > handle the positive and negative deltas separately. > > But later commit b4536f0c829c ("mm, memcg: fix the active list aging > for lowmem requests when memcg is enabled") already removed the LRU > emptiness check, so handling the deltas separately is no longer > needed. And if we just turn it into an atomic long, underflow isn't a > big issue either, and can be checked at the reader side, which is > called much less frequently than the updater. > > So let's turn the counter into an atomic long and check at the reader > side instead, which has a smaller overhead. The underflow correction > is removed: a massive leak of the LRU size counter would indicate > that something else has gone very wrong, and one should fix that > leaking site instead. Besides, the updater-side sanity check is > unlikely to catch the leaking site anyway: if a folio was removed > without updating the counter while other folios remain on the LRU, > the WARN only triggers much later, from a likely innocent callsite. > > Reviewed-by: Ridong Chen > Signed-off-by: Kairui Song > --- > include/linux/memcontrol.h | 9 +++++++-- > mm/memcontrol.c | 18 +----------------- > 2 files changed, 8 insertions(+), 19 deletions(-) > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h > index 215e2e87f42b..7b89d0cb5f6c 100644 > --- a/include/linux/memcontrol.h > +++ b/include/linux/memcontrol.h > @@ -113,7 +113,7 @@ struct mem_cgroup_per_node { > /* Fields which get updated often at the end. */ > struct lruvec lruvec; > CACHELINE_PADDING(_pad2_); > - unsigned long lru_zone_size[MAX_NR_ZONES][NR_LRU_LISTS]; > + atomic_long_t lru_zone_size[MAX_NR_ZONES][NR_LRU_LISTS]; > struct mem_cgroup_reclaim_iter iter; > > /* > @@ -902,10 +902,15 @@ static inline > unsigned long mem_cgroup_get_zone_lru_size(struct lruvec *lruvec, > enum lru_list lru, int zone_idx) > { > + long val; > struct mem_cgroup_per_node *mz; > > mz = container_of(lruvec, struct mem_cgroup_per_node, lruvec); > - return READ_ONCE(mz->lru_zone_size[zone_idx][lru]); > + val = atomic_long_read(&mz->lru_zone_size[zone_idx][lru]); > + if (WARN_ON_ONCE(val < 0)) > + return 0; > + > + return val; > } > > void __mem_cgroup_handle_over_high(gfp_t gfp_mask); > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 11b85f4b6828..a7572ded56c9 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -1529,28 +1529,12 @@ void mem_cgroup_update_lru_size(struct lruvec *lruvec, enum lru_list lru, > int zid, long nr_pages) > { > struct mem_cgroup_per_node *mz; > - unsigned long *lru_size; > - long size; > > if (mem_cgroup_disabled()) > return; > > mz = container_of(lruvec, struct mem_cgroup_per_node, lruvec); > - lru_size = &mz->lru_zone_size[zid][lru]; > - > - if (nr_pages < 0) > - *lru_size += nr_pages; > - > - size = *lru_size; > - if (WARN_ONCE(size < 0, > - "%s(%p, %d, %ld): lru_size %ld\n", > - __func__, lruvec, lru, nr_pages, size)) { > - VM_BUG_ON(1); > - *lru_size = 0; If it happened, the resetting to 0 is removed, will it cause anything unexpected and different behaviour? mem_cgroup_get_zone_lru_size() just read minus value and always WARN_ON_ONCE() if no new adding to this counter. > - } > - > - if (nr_pages > 0) > - *lru_size += nr_pages; > + atomic_long_add(nr_pages, &mz->lru_zone_size[zid][lru]); > } > > /** > > -- > 2.55.0 > >