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b=XGHeCumidfXtyQlwlHTFHq47UBB7t2V9ZZ1C1JtNFAG4K1i9dk4/jj2eukKjcckjf eeHHvUji2rdbDymzp+D3Sd9ljHbmwegU7IJQ3tnjAcEWa3Q+iIGdcZtEyRxN1d+tUV bYVz6MRaozMEEpt9keoFc7YISYEMa05veEKs9Eqw= Date: Fri, 24 Apr 2026 04:53:34 -0700 From: Andrew Morton To: "Barry Song (Xiaomi)" Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Lance Yang , Xueyuan Chen , Kairui Song , Qi Zheng , Shakeel Butt , wangzicheng , Suren Baghdasaryan , Lei Liu , Matthew Wilcox , Axel Rasmussen , Yuanchu Xie , Wei Xu , Will Deacon Subject: Re: [PATCH] mm/mglru: Use folio_mark_accessed to replace folio_set_active in PF Message-Id: <20260424045334.ce9801ecd7a077bf3d5e4ff1@linux-foundation.org> In-Reply-To: <20260418120233.7162-1-baohua@kernel.org> References: <20260418120233.7162-1-baohua@kernel.org> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 508FB40004 X-Stat-Signature: x9rpn61farfsf5s5byaoqttkkfbr1gh5 X-Rspam-User: X-HE-Tag: 1777031616-376610 X-HE-Meta: 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 nZlycD1A 8i0I9AOcEa41UZmLnJTIxQVAAfD6K9OW8eTbTU8DW1UoSYGvno0GqXVPBTUD6VojQc0tJj5fXg3cYvCt8a3tkRNSfxk6Ac1auKbHz5D4OAm4UKXeVJ5mUyZTlWKZLTLaK1sNLRPH5jCfTw/J4QfWPAgdDVXD5WSJWfNQHCenCEXRXAOcSsLGnQtd+lebTOGCLj4h8tapJ5opNUdHcIc7lRbMf3DNczcpi2Ilooo7v8r7jTmzg4d6k2wAyt0/4PNa6IwVSyoe1FNds5/No+R/z/lTbxDhy+G7DC/0SHfSvovoAV8b+ULpaN+QrB2awVLcSfLbil98V4rnTjG/yaZ7EQRgO0cphYesp8cARqJuo1uD5hBVIG5GX0yvKLgXerZ8hGDrPkLp1T7QbIT3b0PRDJX5KwSJUHNZUn7Z3 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sat, 18 Apr 2026 20:02:33 +0800 "Barry Song (Xiaomi)" wrote: > MGLRU gives high priority to folios mapped in page tables. > As a result, folio_set_active() is invoked for all folios > read during page faults. In practice, however, readahead > can bring in many folios that are never accessed via page > tables. > > A previous attempt by Lei Liu proposed introducing a separate > LRU for readahead[1] to make readahead pages easier to reclaim, > but that approach is likely over-engineered. > > Before commit 4d5d14a01e2c ("mm/mglru: rework workingset > protection"), folios with PG_active were always placed in > the youngest generation, leading to over-protection and > increased refaults. After that commit, PG_active folios > are placed in the second youngest generation, which is > still too optimistic given the presence of readahead. In > contrast, the classic active/inactive scheme is more > conservative. > > This patch switches to folio_mark_accessed(). If > folio_check_references() later detects referenced PTEs, > the folio will be promoted based on the reference flag > set by folio_mark_accessed(). > > The following uses a simple model to demonstrate why the current > code is not ideal. It runs fio-3.42 in a memcg, reading a file in a > strided pattern—4KB every 64KB—to simulate prefaulted pages that may > not be accessed. Are you able to suggest any workloads which might regress? And test for those? > Without the patch, we observed 12883855 file refaults and a very low > bandwidth of 58.5 MiB/s, because prefaulted but unused pages occupy > hot positions, continuously pushing out the real working set and > causing incorrect reclaim. With the patch, we observed 0 refaults > and bandwidth increased to 5078 MiB/s. Wow. And that isn't a crazy workload. > For those who want to try the model on x86, you will need the > following in arch/x86/include/asm/pgtable.h. > > #define arch_wants_old_prefaulted_pte arch_wants_old_prefaulted_pte > static inline bool arch_wants_old_prefaulted_pte(void) > { > return true; > } Can you propose a patch? We can at least toss it in there for testing while we think about it. > --- a/mm/swap.c > +++ b/mm/swap.c > @@ -512,7 +512,7 @@ void folio_add_lru(struct folio *folio) > /* see the comment in lru_gen_folio_seq() */ > if (lru_gen_enabled() && !folio_test_unevictable(folio) && > lru_gen_in_fault() && !(current->flags & PF_MEMALLOC)) > - folio_set_active(folio); > + folio_mark_accessed(folio); > > folio_batch_add_and_move(folio, lru_add); > } lol, I was expecting something larger ;)