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AJvYcCVwc1fjwPIMgaq8YrHWJg8FjAaLI62T/326HX06fql46pEQgiKWerM3R+1wz2GFMg88PiYyyVl2ZA==@kvack.org X-Gm-Message-State: AOJu0YxdS57f1FUuxl3dF800CQWDByrymTrmDt0NfP/t8sfU+LmVADyQ 1bBYOWr2RXMD+nPqNrfSC+BtJUWHBrbA+/lGRU7Do6zCxbJEHlhFAkCjo55jI8AxRkJjpHunYd3 qnJyoitOSDD9BdswunSqXijWq0fDncaobeX2BI6Rt X-Gm-Gg: ASbGncsF8mpTPAj1Y7Rs/RafQcIOa583bcCLTcjaGh1IU96EeHUKKJvNTLa8mMZrZUu wbm/vtxCRzt4WtRH54m3YYtzQJRxKqLa5jGCXY2GN1tNGADWwOMxgOIlXljeOz3YjYtS0G4BqLZ h3ibzlm003AiLjFPFL/sdX5VWYJU6h48HRgGq1nv9uVctEk/J3kP8wgM3B7GBtTDb0HAF6WDPaj ufdzAC/743W/7FhmkAlGXlZtjsU8VUr0WPMg1YF0Wo3CLn1otOm4PBp6g== X-Google-Smtp-Source: AGHT+IEfZcQKWymUsTqqOHgiy81N82VSvnJqKcYZvNDzmsCtUqE3l9TNTQbgDZ9aaqLldDgztdzRQMKqVYJRylhPQfU= X-Received: by 2002:a05:6402:2346:b0:61c:b5f0:7ddb with SMTP id 4fb4d7f45d1cf-6235d1a4bb2mr2464a12.6.1757307398564; Sun, 07 Sep 2025 21:56:38 -0700 (PDT) MIME-Version: 1.0 References: In-Reply-To: From: Lokesh Gidra Date: Sun, 7 Sep 2025 21:56:26 -0700 X-Gm-Features: Ac12FXz21g3aZY8pBxInKDKKJwg-ZQSv234aSGthQCUMDFh06_0DIrztZfN_DM8 Message-ID: Subject: Re: [DISCUSSION] Unconditionally lock folios when calling rmap_walk() To: David Hildenbrand Cc: Lorenzo Stoakes , 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. Howlett" Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 8361680002 X-Stat-Signature: k6begk7s5ekpcm6w8e3yxxxzb68oa75p X-Rspam-User: X-HE-Tag: 1757307400-651172 X-HE-Meta: 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 xQdRO0dR WhMDCe+oyftUVTz/EVvCKF0MlyWqKtCSm0oIUT/a0d/lu4+GGgJnIgASes1bGfIsaYOuY4X4tgkf8/sv+ZC9z/uOsjKGnjk9r9ZiezX1o9UpNFQ3sIEcvfDRzzwyRyZX9D3gWSD6HyHhMY3IfAqs8jhcM9anTHuKEO4WmGN7VCGY+4jKMzza+q7U6KPvfmIN7lDAvR0HGnKf4UJ8S3dBApYupBsRVztp7uJsNpR1+QxtJK03Gg5HLPuEwpu5CxozrGhBvOgKHC1vDF+z9jkLuw93m8yl1M9XaCLRxuFZQo1Oll2nMvXI+AHGr+Ffhg7KAG7WPj4Bu40aTY/c= 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 Tue, Sep 2, 2025 at 12:04=E2=80=AFPM Lokesh Gidra wrote: > > On Tue, Sep 2, 2025 at 12:01=E2=80=AFPM David Hildenbrand wrote: > > > > On 02.09.25 20:59, Lokesh Gidra wrote: > > > On Fri, Aug 29, 2025 at 2:04=E2=80=AFAM 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 differ= ently. > > >>>>> > > >>>>> Perhaps even saying 'try and get the rmap lock, but if there's "t= oo 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 hel= p > > >>>> (in terms of avoiding contention) if we were to change the conditi= on: > > >>> > > >>> I think we shouldn't be using PAE here. Once could consider using > > >>> folio_maybe_mapped_shared(), and assume contention on the folio loc= k if it > > >>> is maybe mapped shared. > > >> > > >> Interesting! > > >> > > >>> > > >>> But the real question is with whom we would be contending for the f= olio > > >>> 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 propos= ed 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. :) > > Sent the patches: https://lore.kernel.org/all/20250908044950.311548-1-lokeshgidra@google.com/ > Sounds good. Will do. Thanks. > > -- > > Cheers > > > > David / dhildenb > >