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(p200300d82f1f3f00731af5e5774ed40c.dip0.t-ipconnect.de. [2003:d8:2f1f:3f00:731a:f5e5:774e:d40c]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45b9a6ecfafsm35391745e9.21.2025.09.02.12.01.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 02 Sep 2025 12:01:39 -0700 (PDT) Message-ID: Date: Tue, 2 Sep 2025 21:01:38 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [DISCUSSION] Unconditionally lock folios when calling rmap_walk() To: Lokesh Gidra , Lorenzo Stoakes Cc: Andrew Morton , Harry Yoo , Zi Yan , Barry Song <21cnbao@gmail.com>, "open list:MEMORY MANAGEMENT" , Peter Xu , Suren Baghdasaryan , Kalesh Singh , android-mm , linux-kernel , Jann Horn , Rik van Riel , Vlastimil Babka , "Liam R. 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 3447340024 X-Stat-Signature: tazqo5qgxa3dj9san7gwsqmt1hnsghqs X-Rspam-User: X-HE-Tag: 1756839708-803688 X-HE-Meta: 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 09EjD1bx 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 02.09.25 20:59, Lokesh Gidra wrote: > On Fri, Aug 29, 2025 at 2:04 AM Lorenzo Stoakes > wrote: >> >> On Fri, Aug 29, 2025 at 10:42:45AM +0200, David Hildenbrand wrote: >>>>> >>>>> I do wonder if we can identify this case and handle things differently. >>>>> >>>>> Perhaps even saying 'try and get the rmap lock, but if there's "too much" >>>>> contention, grab the folio lock. >>>> >>>> Can you please elaborate what you mean? Where do you mean we can >>>> possibly do something like this? >>>> >>>> UFFD move only works on PageAnonExclusive folios. So, would it help >>>> (in terms of avoiding contention) if we were to change the condition: >>> >>> I think we shouldn't be using PAE here. Once could consider using >>> folio_maybe_mapped_shared(), and assume contention on the folio lock if it >>> is maybe mapped shared. >> >> Interesting! >> >>> >>> But the real question is with whom we would be contending for the folio >>> lock. >>> >>> Is it really other processes mapping that folio? I'm not so sure. >> >> Yeah, I might go off and do some research myself on this, actually. Nail down >> wehre this might actually happen. >> >> Generally I'm softening on this and maybe we're good with the proposed change. >> >> But still want to be super careful here... :) >> > Anxiously waiting for your assessment. Fingers crossed :) I'd suggest you prepare an RFC patch where you neatly summarize all we learned so far. :) -- Cheers David / dhildenb