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Howlett" To: Peter Zijlstra Cc: Suren Baghdasaryan , akpm@linux-foundation.org, willy@infradead.org, lorenzo.stoakes@oracle.com, mhocko@suse.com, vbabka@suse.cz, hannes@cmpxchg.org, mjguzik@gmail.com, oliver.sang@intel.com, mgorman@techsingularity.net, david@redhat.com, peterx@redhat.com, oleg@redhat.com, dave@stgolabs.net, paulmck@kernel.org, brauner@kernel.org, dhowells@redhat.com, hdanton@sina.com, hughd@google.com, lokeshgidra@google.com, minchan@google.com, jannh@google.com, shakeel.butt@linux.dev, souravpanda@google.com, pasha.tatashin@soleen.com, klarasmodin@gmail.com, corbet@lwn.net, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH v6 10/16] mm: replace vm_lock and detached flag with a reference count Message-ID: <6nck2rfwcytqdinsavmewytgcca43mldlczmao3zztrpr5v2ci@4xn6nwp6tcih> Mail-Followup-To: "Liam R. 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 m9wrjtBS 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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: * Peter Zijlstra [241219 13:47]: > On Thu, Dec 19, 2024 at 01:18:23PM -0500, Liam R. Howlett wrote: > > > > For RCU lookups only the mas tree matters -- and its left present there. > > > > > > If you really want to block RCU readers, I would suggest punching a hole > > > in the mm_mt. All the traditional code won't notice anyway, this is all > > > with mmap_lock held for writing. > > > > We don't want to block all rcu readers, we want to block the rcu readers > > that would see the problem - that is, anyone trying to read a particular > > area. > > > > Right now we can page fault in unpopulated vmas while writing other vmas > > to the tree. We are also moving more users to rcu reading to use the > > vmas they need without waiting on writes to finish. > > > > Maybe I don't understand your suggestion, but I would think punching a > > hole would lose this advantage? > > My suggestion was to remove the range stuck in mas_detach from mm_mt. > That is exactly the affected range, no? Yes. But then looping over the vmas will show a gap where there should not be a gap. If we stop rcu readers entirely we lose the advantage. This is exactly the issue that the locking dance was working around :)