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AJvYcCUYDJtX4bNIYBiUt9ngDLZ3Rja5Jd26MovuMVa56VQyFhoM+S6zy+m5dt2OW/2HeGdhuni0RZ4u9w==@kvack.org X-Gm-Message-State: AOJu0YwsGhw0oBDd3UJT5I3jTQ0HRVRuUdB6v9O5a1e94UM1e2fQ+ghS BVRgpFX0mN2M/8edbLBsX6cAecyQNT72nDg8u4qJYEpmwx/XJd2Ihte28yQgdUuUH2HKOWh+NK9 0rh+Wdq6bVli//cFq6t7B5LiC0ncWNT0= X-Gm-Gg: ASbGnctcAupWq/LE3pGvktwx+QwkWXOfFF9AgPyAnm7ZCLw/A3YQlAT8BpXdoKSNtE3 F0A4imwsJYSHGh5STf5/KijX29Y7I8VliVF4Not+YeX3kTdGgy3IZq7sOfnYPv+TKSrjadCdPbG d7tegGnd08JjGFHDWNnRdbeo81a1hNKMkPMijT1c91FMlZ8rCzv/SDrtAvZHLNnNko9O9dqksnw KDjeqxP1uVu/X+c4bxsPk4J4n3rCYNwXUZDGXvo1LdPlhX1a5jQTNoSkmvy X-Google-Smtp-Source: AGHT+IHr0N2c9rpkUD+j96nPJboavaEVvbPlab1UDSv2Npm0903BsJX6fJADk+MWL+Xob2pbB0iwVGgwn464ObRlNfA= X-Received: by 2002:a05:620a:2909:b0:7fa:aea2:24f8 with SMTP id af79cd13be357-813bf6c1877mr2115024485a.23.1757582309901; Thu, 11 Sep 2025 02:18:29 -0700 (PDT) MIME-Version: 1.0 References: In-Reply-To: From: Barry Song <21cnbao@gmail.com> Date: Thu, 11 Sep 2025 21:18:17 +1200 X-Gm-Features: Ac12FXyP1xyQ_RoyxPc1P9Mzf30DhbDFb160kQ2sCsUBiaNPu2_Hbdxvx7e4TzU Message-ID: Subject: Re: [DISCUSSION] anon_vma root lock contention and per anon_vma lock To: David Hildenbrand Cc: Nicolas Geoffray , Lokesh Gidra , Lorenzo Stoakes , Harry Yoo , Suren Baghdasaryan , Andrew Morton , Rik van Riel , "Liam R . Howlett" , Vlastimil Babka , Jann Horn , Linux-MM , Kalesh Singh , SeongJae Park , Barry Song , Peter Xu Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: E6ED880005 X-Stat-Signature: rajtypeagzwrsz1y1kmzjzsc1nus6hkr X-Rspam-User: X-HE-Tag: 1757582310-748986 X-HE-Meta: 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 qfhffAHk ZsQfanTHiauYNoH3LESs6gP5MUC13h9izn3oiOdpkhhSh63VjWy1XK64CK9yTnTsdTW6rHFFluDSeVDLvD537hav7dEATZQ79JBMRZYvNcvCkI/L0n6yrjQRKlP6WvPj+rG2ObQ7a1Jc+arrHkhAx2/O+I+XKhZvC25CURp1/U3koCdh/R11RQeP/DO5mLAIngio5G5UOwMHutAuZ08Vkn6oPliQ6j9dbl3dMiMUc2Ajt47DO0Am6CQploNT4e7l5PDnQkikBG1zbjwjescxFctj8B4UHjWaNlCsKSwzMMDZnjI3jfJO+lWhtYm6pwXlQatCsUt4lItzHTt1+AjOjuHNXkD5LL8T9V8CpvCGL1UjQgX6yO0oRAXpmUZCUaGCxBXlx2HtaxMfyCiNsBVPlJCdlHbq2deA4fwZ+GoyCkt4IaX0= 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 Thu, Sep 11, 2025 at 8:14=E2=80=AFPM David Hildenbrand wrote: > > On 11.09.25 09:17, Barry Song wrote: > > Hi All, > > > > I=E2=80=99m aware that Lokesh started a discussion on the concurrency i= ssue > > between usefaultfd_move and memory reclamation [1]. However, my > > concern is different, so I=E2=80=99m starting a separate discussion. > > > > In the process tree, many processes may share anon_vma->root, even if > > they don=E2=80=99t share the anon_vma itself. This causes serious lock = contention > > between memory reclamation (which calls folio_referenced and try_to_unm= ap) > > and other processes calling fork(), exit(), mprotect(), etc. > > > > On Android, this issue becomes more severe since many processes are > > descendants of zygote. > > > > Memory reclamation path: > > folio_lock_anon_vma_read > > > > mprotect path: > > mprotect > > split_vma > > anon_vma_clone > > > > fork / copy_process path: > > copy_process > > dup_mmap > > anon_vma_fork > > > > exit path: > > exit_mmap > > free_pgtables > > unlink_anon_vmas > > > > To be honest, memory reclamation=E2=80=94especially folio_referenced()= =E2=80=94is a > > problem. It is called very frequently and can block other important > > user threads waiting for the anon_vma root lock, causing UI lag. > > > > I have a rough idea: since the vast majority of anon folios are actuall= y > > exclusive (I observed almost 98% of Android anon folios fall into this > > category), they don=E2=80=99t need to iterate the anon_vma tree. They b= elong to > > a single process, and even for rmap, it is per-process. > > > > I propose introducing a per-anon_vma lock. For exclusive folios whose > > anon_vma is not shared, we could use this per-anon_vma lock. > > folio_referenced declares that it will begin reading, and Lokesh=E2=80= =99s > > folio_lock may also help maintain folios as exclusive, so I am > > somewhat in favor of his RFC. Any thread writing to such an anon_vma > > would take the per-vma write lock, and possibly also the anon_vma > > root write lock. If folio_referenced fails to declare the per-vma lock, > > it can fall back to the global anon_vma->root read mutex, similar to > > mmap_lock. > > To summarize, are you proposing a similar locking scheme like we have > for mm vs. vma here for anon-vma root vs. anon-vma? Quite similar, but with the optimization limited only to exclusive anon folios. The main issue is in folio_referenced(), which frequently takes the anon_vma root read lock. Complaints are likely due to memory reclamation holding this read lock=E2=80=94if the process is preempted, it becomes runnable but not running, while fork(), mprotect(), and exit() may be forced to wait. I haven=E2=80=99t seen complaints about writer=E2=80=93writer contention, so I don=E2=80=99t plan to optimize write= -side conflicts at this stage. In short: the problem is frequent rwsem read locks that get preempted, blocking fork(), mprotect(), and exit(). If a folio is exclusive and we already hold folio_lock, it should remain exclusive to a single process and a single vma. In that case, such folios may not need an rmap tree at all for folio_referenced(). I=E2=80=99m mainly concerned about cases where a read lock is held but neve= r a write lock. As long as a folio is exclusive, stays exclusive during rmap, and its rmap node remains present, there=E2=80=99s no need to modify the rm= ap tree. When changes are made, both the target being modified and the anon_vma->root should be locked. We are not altering writer behavior by requiring the anon_vma->root lock. In short, I=E2=80=99m seeking a simple way to avoid taking anon_vma->root during memory reclamation for folios mapped exclusively by a single process. Thanks Barry