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Tue, 08 Sep 2026 08:23:40 +0000 X-Mizu-Trace-ID: 99ae5adff9099bc7 X-Migadu-Flow: FLOW_OUT Date: Tue, 8 Sep 2026 16:23:32 +0800 From: Baoquan He To: Baolin Wang Cc: Barry Song , akpm@linux-foundation.org, kasong@tencent.com, qi.zheng@linux.dev, shakeel.butt@linux.dev, 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, hebaoquan@kylinos.cn, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: mglru: clear the reference counter for rejected folios Message-ID: References: <8e4db9a298c5ea6ccb192e274caed5b96f0cf022.1788751143.git.baolin.wang@linux.alibaba.com> <67d9bbcf-2991-4867-8afb-14bad1cebf8d@linux.alibaba.com> <31494361-406e-4874-8a2c-2c6f5cee5321@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 3A0854000D X-Stat-Signature: y5xj81qgnkx4qsquscwepooy9wcz3k8b X-Rspam-User: X-HE-Tag: 1788855821-829284 X-HE-Meta: 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 4w/naY/s aKd20jvGNQr3YS1gQx39dyHCj0Htc/eKUkvG/fiZve90EZXoim36RrxKlwNJwfnAzjdUzwAmikkTrefgSJwlLTZoncIEDP/SSn5gRjUzjWPFJdoiK063+RofQT3KNXO7NvaPL8O5zYWAWk1w7nT/c5FyQMBbkyxNzTthvHnrLuBGL8OCy/P2fUxIDs+9cn7zdZAjwiNcth130vvuHMaujAl5kb/ivj5Z9vpRNzMFkb00IHUvGdyR5LQh3rupUlkhwjNcqWbW743RALIHlZxBsuoRskUgbECA9vJpUDWcP5r++SZRQiqgkeIkPWbvB6AIcflPt Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 09/08/26 at 03:53pm, Baolin Wang wrote: > > > On 9/8/26 2:59 PM, Baoquan He wrote: > > On 09/08/26 at 12:01pm, Baolin Wang wrote: > > > > > > > > > On 9/8/26 11:03 AM, Baolin Wang wrote: > > > > > > > > > > > > On 9/8/26 10:34 AM, Barry Song wrote: > > > > > On Tue, Sep 8, 2026 at 10:30 AM Baoquan He wrote: > > > > > > > > > > > > Hi Baolin, > > > > > > > > > > > > On 09/07/26 at 11:25am, Baolin Wang wrote: > > > > > > ......snip... > > > > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c > > > > > > > index 40d3f1b48a74..42c0a09938ab 100644 > > > > > > > --- a/mm/vmscan.c > > > > > > > +++ b/mm/vmscan.c > > > > > > > > > > > > Well, this seems to be based on Andrew's mm-new branch. I usually track > > > > > > mm-unstable branch. Maybe the subject should be marked as below? > > > > > > [PATCH mm-new] mm: mglru: clear the reference counter for rejected > > > > > > > > ACK. > > > > > > > > > > > > > > > > > @@ -5021,10 +5021,11 @@ static int evict_folios(unsigned > > > > > > > long nr_to_scan, struct lruvec *lruvec, > > > > > > >                } > > > > > > > > > > > > > >                /* 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); > > > > > > > -             } > > > > > > > + > > > > > > > +             /* See the comments on LRU_REFS_FLAGS */ > > > > > > > +             folio_set_lru_refs(folio, 0); > > > > > > > > > > > > This looks like a great catch, while the code change could bring issue. > > > > > > > > > > > > Because move_folios_to_lru() relies on folios' flags to decide their new > > > > > > generation. You just cleared it before move_folios_to_lru(). This is no > > > > > > problem for rejected folios that are determined to be put into the > > > > > > oldest generation. But for those rejected folios that are determined to > > > > > > be promoted, this could be wrong. E.g currently gen window is 4, and a > > > > > > folio is referenced, lru_gen_folio_seq() decides its new gen as 1, which > > > > > > is the 2nd oldest generation. While folio_set_lru_refs(folio, 0) clear > > > > > > referenced bit, this causes it being put into the oldest generation in > > > > > > move_folios_to_lru(), this is not expected. > > > > > > > > Yes. As I discussed with Barry earlier, lru_gen_folio_seq() also needs > > > > to be reconsidered regarding whether it should rely on PG_referenced > > > > [1]. > > > > > > > > For commit 6cbdd9726fb5, we didn't discuss the impact on rejected folios > > > > either. Before commit 6cbdd9726fb5, if rejected folios did not have > > > > PG_active set by shrink_folio_list(), evict_folios() would set PG_active > > > > on these rejected folios. > > > > > > > > [1] https://lore.kernel.org/linux-mm/20260901220430.79810-1- > > > > baohua@kernel.org/ > > > > > > > > > The original code looks quite weird. It even prioritizes folios > > > > > that won't be promoted by `PG_active`. Do we need to change all > > > > > the cases just to call `PG_active`? > > > > I would agree if we can. While I have one concern. A rejected folio that > > should have gone into the oldest generation is now being promoted to the > > 2nd newest generation. In the original code, referenced folio is only > > being promoted to the next gen. I even think this is not a bug, but Yu > > That's not quite true. Before commit 6cbdd9726fb5, a rejected referenced > folio was also put back to the 2nd youngest gen. OK, I didn't follow your earlier discussion, I need take some time to fully understand that commit and the patch thread from Ehab. > > > Zhao intentionally did it: the coldest folio is moved to 2nd newest gen, > > referenced folio (hot folio) is moved to new gen but carries the referenced > > bit. Both of them seems to be treated somewhat equally. > > > > To me, I would rather move both of them to the next gen, while keep their > > refs untouched. > > IMHO, I strongly recommend not doing this, and that's exactly the motivation > behind my patch. Because this is also being treated as a promotion, it > should behave like folio_inc_gen() or folio_update_gen() and clear the refs > after promotion. I think this is a fundamental principle of promotion. > Otherwise, ref-based promotion is already completely broken. OK, it makes sense to me to make principle of promotion strictly applied no efficiency degradation involved. > > Next, I also plan to clean up refs in lru_gen_set_refs() as discussed with > Barry. Looks forward to seeing that. By the way, your discussion with Baryr is private or in public list, do you have pointer if public? Thanks. > > > > > Yes. Regarding this concern, I plan to change back to the original > > > > behavior: > > > > > > > >     /* 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_active(folio); > > > > > > > > What do you think? > > > > > > Just FYI, after above changes, the performance improvement on zram is no > > > longer obvious either. I think this also answers Kairui's earlier question > > > about why I saw a performance improvement (which seems related to commit > > > 6cbdd9726fb5). Also, there is no obvious performance regression either. > > > > Hi Baolin, > > > > Not sure if it's convenient to do a little more testing in your side. > > E.g rejected folios are moved to next gen, but not clearing their flags. > > I'm not sure what you mean by "next gen" here. If you mean the 2nd oldest > gen, I actually tested that too, that is, clearing refs after > move_folios_to_lru(), and there wasn't any noticeable performance impact > either (but the code was a bit hacky, so I didn't go with this approach). Yeah, I meant the 2nd oldest gen, while what I am curious about is moving them into 2nd oldest gen but not clearing refs. Clearly it's conflicting with your plan. Anyway, it's just a brain store idea, please forget it. Thanks for the sharing and detailed explanation. > > > Then we can at least know which one is more efficient on yout testing > > platform. > > > > coldest folio referenced folio > > --------------------------------------------- > > max_gen -1 next gen + referenced kept > > max_gen -1 max_gen -1 > > next_gen next gen + referenced kept > > > > Thanks > > Baoquan >