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Wed, 09 Sep 2026 03:11:36 +0000 X-Mizu-Trace-ID: aba1175d2e6d8a11 X-Migadu-Flow: FLOW_OUT Date: Wed, 9 Sep 2026 11:11:33 +0800 From: Baoquan He To: Baolin Wang Cc: akpm@linux-foundation.org, kasong@tencent.com, qi.zheng@linux.dev, shakeel.butt@linux.dev, baohua@kernel.org, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, hannes@cmpxchg.org, david@kernel.org, mhocko@kernel.org, ljs@kernel.org, ridong.chen@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH mm-new v2] mm: mglru: clear the reference counter for rejected folios Message-ID: References: <9214e36bf738fcfba86acc8cea85dff4010f66b0.1788918714.git.baolin.wang@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9214e36bf738fcfba86acc8cea85dff4010f66b0.1788918714.git.baolin.wang@linux.alibaba.com> X-Rspam-User: X-Rspamd-Queue-Id: BB31320003 X-Stat-Signature: pjs7qrzrkm1iuauoiju3assyr7cbx84w X-Rspamd-Server: rspam01 X-HE-Tag: 1788923499-218274 X-HE-Meta: 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 bVxunbNh MMJrFk/ijwZoIew4R5SIGA3ijCrXQz/lKu7H05v8xBU97xNvBGICp9bhcygSO/AhaeaL+T3OtdBFrgurZ72cc5bnvigUE8FDXZWvYQ8nyANGyBnq/vEAYTVdT1il++p3f0Lep7ycHhaCkJKqfWDI8RH/ATaYG2iCKqxn/PU4hFRVeW8KJtPqnT0VK1k5/N6p7exsQ97qiGzfgTHHx5sGUcywSlPbII/NDIO4ExHJrrpmqpbhK2x0UaCIAdtFaDEOq4DvUKyJS+Hydks4Qx40SytDQRlAoeEQ0U6Fa Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 09/09/26 at 09:57am, Baolin Wang wrote: > As per the comment on LRU_REFS_FLAGS, when accessed folios are promoted to > a new generation, LRU_REFS_FLAGS should be cleared so that the reference > counter can start over. > > For folios rejected by shrink_folio_list(), we clear LRU_REFS_FLAGS and > set the PG_active flag when lru_gen_folio_seq() would place them in the > oldest generation. That's fine. > > But for rejected folios where lru_gen_folio_seq() returns a generation > other than the oldest one (which can be treated as a promotion), we do > not clear LRU_REFS_FLAGS. This can violate the promotion mechanism. And > this means the rejected folio enters the new generation with stale, inflated > tier bits, which can inflate reference counts and distort eviction > statistics for these rejected folios. > > Fix this by clearing LRU_REFS_FLAGS for rejected folios. Of course, I need > to evaluate the impact of the changes, which mainly falls into 3 cases: > 1. When lru_gen_folio_seq() returns the oldest generation for rejected folios, > there are no logic changes, and they will be put back into the 2nd youngest > generation. > 2. For rejected folios with PG_active set by shrink_folio_list(), we only > clear the LRU_REFS_FLAGS and do not change the generation. > 3. For rejected folios with PG_referenced set, the original code would put > them back into the 2nd oldest generation. After this patch, we will put them > back into the 2nd youngest generation. > > I think case 3 is also reasonable, before commmit 6cbdd9726fb5 ("mm/mglru: > use folio_mark_accessed to replace folio_set_active"), a rejected referenced > folio was also put back to the 2nd youngest gen. Meanwhile, I didn't see > any noticeable performance impact on my 32-core Arm machine when running > 'make -j32' to build the kernel inside a 3G-limited memcg with either zram > or NVMe swap. According to our discussion in v1 thread, the patch log clearly states the justification, and the code change looks good to me. Thanks for the effort. Reviewed-by: Baoquan He > > Signed-off-by: Baolin Wang > --- > Changes from v1: > - Update the commit message. > - Clear LRU_REFS_FLAGS earlier. > --- > mm/vmscan.c | 7 ++++--- > 1 file changed, 4 insertions(+), 3 deletions(-) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 40d3f1b48a74..1557679aadb1 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -5020,11 +5020,12 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec, > continue; > } > > + /* See the comments on LRU_REFS_FLAGS */ > + folio_set_lru_refs(folio, 0); > + > /* don't add rejected folios to the oldest generation */ > - if (lru_gen_folio_seq(lruvec, folio, false) == min_seq[type]) { > - folio_set_lru_refs(folio, 0); > + if (lru_gen_folio_seq(lruvec, folio, false) == min_seq[type]) > folio_set_active(folio); > - } > } > > move_folios_to_lru(&list); > -- > 2.47.3 >