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([43.153.32.141]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3207ebaefb1sm2257252a91.8.2025.07.31.07.56.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 31 Jul 2025 07:56:41 -0700 (PDT) From: Jinliang Zheng X-Google-Original-From: Jinliang Zheng To: hch@infradead.org Cc: alexjlzheng@gmail.com, alexjlzheng@tencent.com, brauner@kernel.org, djwong@kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org, willy@infradead.org Subject: Re: [PATCH v2] iomap: move prefaulting out of hot write path Date: Thu, 31 Jul 2025 22:56:40 +0800 Message-ID: <20250731145640.651097-1-alexjlzheng@tencent.com> X-Mailer: git-send-email 2.49.0 In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-xfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Thu, 31 Jul 2025 07:21:57 -0700, Christoph Hellwig wrote: > On Thu, Jul 32, 2025 at 12:44:09AM +0800, alexjlzheng@gmail.com wrote: > > From: Jinliang Zheng > > > > Prefaulting the write source buffer incurs an extra userspace access > > in the common fast path. Make iomap_write_iter() consistent with > > generic_perform_write(): only touch userspace an extra time when > > copy_folio_from_iter_atomic() has failed to make progress. > > This is probably a good thing to have, but I'm curous if you did see > it making a different for workloads? Yes, there is some improvement. However, I tested it only a few times, so I can't rule out jitter. However, from a design pattern perspective, this patch is a good thing anyway. > > > + /* > > + * Faults here on mmap()s can recurse into arbitrary > > + * filesystem code. Lots of locks are held that can > > + * deadlock. Use an atomic copy to avoid deadlocking > > + * in page fault handling. > > We can and should use all 80 characters in a line for comments. I agree. hahaha :) thanks, Jinliang Zheng. :) :) > > > + /* > > + * 'folio' is now unlocked and faults on it can be > > + * handled. Ensure forward progress by trying to > > + * fault it in now. > > + */ > > Same here. > > I really wish we could find a way to share the core write loop between > at least iomap and generic_perform_write and maybe also the other copy > and pasters. But that's for another time..