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Received: from willy by casper.infradead.org with local (Exim 4.99.1 #2 (Red Hat Linux)) id 1wsbcQ-00000004vh6-15Hy; Sat, 08 Aug 2026 07:37:22 +0000 Date: Sat, 8 Aug 2026 08:37:22 +0100 From: Matthew Wilcox To: Suren Baghdasaryan Cc: akpm@linux-foundation.org, dave.hansen@linux.intel.com, Liam.Howlett@oracle.com, ljs@kernel.org, david@kernel.org, shakeel.butt@linux.dev, vbabka@kernel.org, jannh@google.com, aliceryhl@google.com, arve@android.com, cmllamas@google.com, christian@brauner.io, tkjos@android.com, dsahern@kernel.org, davem@davemloft.net, gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, netdev@vger.kernel.org Subject: Re: [PATCH v4 3/5] mm: Add RCU-based VMA lookup helper that waits for writers Message-ID: References: <20260806200548.3124802-1-surenb@google.com> <20260806200548.3124802-4-surenb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260806200548.3124802-4-surenb@google.com> X-Rspamd-Server: rspam07 X-Rspam-User: X-Stat-Signature: 856zb1od6ufi5ykhb1efj59kr4fnrhd5 X-Rspamd-Queue-Id: E56EA1C0008 X-HE-Tag: 1786179724-631682 X-HE-Meta: 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 Jkk659c2 ElY3OJdDfOzAcztmURRedcXbNxLY6Izp48SabsBZHMaKKuWzqcKfo7CCWyh5uqzTvgV4iZ+DQg97DNlJd97woMyNWSKBdMN5N/j52Belb1f+6ddFnAXnSvPArhcEOtElKGigTuGtTRucXCoZ378NsUPGh76G8kif85VnmJSC/GG2KkT48JvFFTYPV8fDfjVIwcwSOiHGdIBZexrnx+BeBgkgjcOAPETfizZwp7PwvF5wkSfkCRixCB03LT451vZyolywlcvYnFBfzqQGfz10rIGyxebMvqnMGaFTA5ERwpVX7CDIlcKGNJWkvnqkYfXuZRjL2it2hCGPIsVky/0H7kYVLow== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Aug 06, 2026 at 01:05:46PM -0700, Suren Baghdasaryan wrote: > From: Dave Hansen > > == Background == > > There are basically two parallel ways to look up a VMA: the > traditional way, which is protected by mmap_read_lock, and the RCU-based > per-VMA lock way which is based on RCU and refcounts. > > == Problem == > > The mmap_lock one is more straightforward to use but it has a big > disadvantage in that it can not be mixed with page faults since those > can take mmap_lock for read, which can deadlock when mixed with nested > page faults and parallel writers. > For example: > > mmap_read_lock(mm); > // Another thread does mmap_write_lock(). > // New mmap_lock readers are blocked. > vma = vma_lookup(mm, address); > // This deadlocks on mmap_read_lock() if it faults: > copy_from_user(address); > mmap_read_unlock(mm); > > The per-VMA lock can be mixed with faults, but they can fail and need to > be able to fall back to the traditional way. > > == Solution == > > Add vma_start_read_unlocked() - a variant of the RCU-based lookup that > waits for writers. This is basically the same as the existing RCU-based > lookup, but on a failure to lock it temporarily takes mmap_lock for read > and waits for writers to finish before locking the VMA, dropping the > mmap_lock and returning the locked VMA. This has some advantages: > > 1. Callers do not need to have a fallback path for when they > collide with writers. > 2. It can be used in contexts where page faults can happen because > it can take the mmap_lock for read but never *holds* it. > 3. Its fast path does not require taking mmap_lock for read. > > Basically, when applied correctly, this approach results in faster > *and* simpler code. Maybe just write a new commit message, because while this is a useful primitive to have, (as evidenced by patch 5) it doesn't do what this commit message claims.