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Tue, 08 Sep 2026 10:29:35 +0000 X-Mizu-Trace-ID: 965e93f6bc2a3c87 X-Migadu-Flow: FLOW_OUT Date: Tue, 8 Sep 2026 18:29:28 +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> <7d2ee486-0f59-43b7-8449-fed82b796a73@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <7d2ee486-0f59-43b7-8449-fed82b796a73@linux.alibaba.com> X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 2D105A0005 X-Stat-Signature: ma1r9wh8bcyefzxt6qikz54h8wbnwe3d X-Rspam-User: X-HE-Tag: 1788863376-72159 X-HE-Meta: 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 6+r2V6hE X93bxqSeRPkGRHipkPRy6NNVRvrRujegwq+snIBNTeW4OT2sZ9+44ivx7FXsv4OPUtIDqVHcD84422GaeKD3UTOw1wMNzjxUM4R2EyhinNMipJNtZfuA5IucI/tfv1QPhJYH8KS4R9UZT9iQACO01RDHcfTemUJJZPvBsmYbjxfyKuHUBmIfIJPdsfoITWPAdwWYC1eBut0hilH4YkiyWKH+pbzVH1fffAaiyRTqWgec41g3JPm7PvrGTTcf9y/PdUBDrhMZmqa4sburAFN6WqaHIvoHocPrvJdKTPFbDe7BChHjvnDfdFJxWszKx2kA/AvCTaqlTPTw+BVn+VwI2x2sVRw== 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 05:39pm, Baolin Wang wrote: > > > On 9/8/26 4:23 PM, Baoquan He wrote: > > 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. > > I raised this issue before[1], and recently there has been another > discussion[2] about it (we had some private discussions, but I'll post a new > patch for discussion). > > [1] https://lore.kernel.org/all/eb395442-0aad-428a-a5ac-9072d2d89060@linux.alibaba.com/ > [2] > https://lore.kernel.org/linux-mm/20260901220430.79810-1-baohua@kernel.org/ Got it, 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. > > Thanks for reviewing. >