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Sun, 07 Jun 2026 20:32:09 -0700 (PDT) From: Cunlong Li To: SeongJae Park Cc: Andrew Morton , damon@lists.linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/damon/vaddr: drop redundant mmap_read_lock from damon_va_three_regions Date: Mon, 8 Jun 2026 11:32:05 +0800 Message-Id: X-Mailer: git-send-email 2.30.2 In-Reply-To: <20260607173644.93883-1-sj@kernel.org> References: <20260607-rcu-v1-1-3c9f5995a3a0@gmail.com> <20260607173644.93883-1-sj@kernel.org> Mail-Followup-To: Cunlong Li , SeongJae Park , Andrew Morton , damon@lists.linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 7CBCD140003 X-Stat-Signature: uw3wc833bhmf99htxszmogyc5f1u3gpr X-HE-Tag: 1780889531-10663 X-HE-Meta: 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 n+kNxVT0 gvT+hiWdLgtvZ6xlOLEg4/tf/tkegIIJTsHB9jve4WF7PaHKhmaJMtbAjRNxGcdjQJ8g4fWoUEnWaqhxH2Kr4czkJ+HcMGoOTgNeMbfSwcXBYy9/im1Ak4l7OWpOuEP429bGR6UVhnJxqek+XDQoLc2sn5xlLe7JcBj3VGgyPPKc6QW0dyzW6+qNBdZS4FjyGcdlqyYAR7/Fd36S2CurkLX9Zbcf8otZdeMoitaqsjtU1KJ7M+HIVPHOsMJIikQKpEhNx+idH8+RPA0xUEJL9gXbzMYe/dX4yxb/95p5l6pLpIzcMbgT4uGzy9cSfDTNOsKdrGmuNxZ6sanEw14pWM0ZmqO3hNfL7wEFNtCpF2BVHnB6q6WhxczHBaE6965ds6BPK88gzCMyY6d0ZYJEFnZeGY93+qnmbMl46owCxGCtBX4mC31faYfX4p9wiTEFj4SLnnezHa6EpSvzaHRR0OHXWyA81Hp4FY0fibfQsKacpKoY= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sun, Jun 07, 2026 at 10:36:44AM -0700, SeongJae Park wrote: > Hello Cunlong, > > On Sun, 07 Jun 2026 21:48:52 +0800 Cunlong Li wrote: > > > __damon_va_three_regions() already walks VMAs under rcu_read_lock(), so > > the outer mmap_read_lock() is no longer needed. > > The function aims to read a snapshot of the virtual address space. So I think > mmap_read_lock() is still required. Please let me know if I'm missing > something. > > > Thanks, > SJ > > [...] Yes — I understand this path is meant to obtain a snapshot of the VMA layout. My thinking was that an exact snapshot may not be required here, and the RCU-protected traversal inside __damon_va_three_regions() already makes the walk safe, so the outer mmap_read_lock() might be redundant. But, if we want a consistent VMA layout here, mmap_read_lock() is the right mechanism and rcu_read_lock() alone is not enough. I'll drop the patch :) Thanks, Cunlong