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Wed, 15 Apr 2026 06:05:28 -0400 (EDT) Date: Wed, 15 Apr 2026 11:05:21 +0100 From: Kiryl Shutsemau To: Mingyu He Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, hannes@cmpxchg.org, clm@meta.com, linux-kernel@vger.kernel.org, willy@infradead.org, bfoster@redhat.com, Jens Axboe , littleswimmingwhale@gmail.com Subject: Re: [ISSUE] Read performance regression when using RWF_DONTCACHE form 8026e49 "mm/filemap: add read support for RWF_DONTCACHE" Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: ADDD2C000B X-Stat-Signature: xmc5msd3rqnm3ufzbngn6edaonyng8ak X-HE-Tag: 1776247531-177907 X-HE-Meta: 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 l3jpWl6A 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Apr 15, 2026 at 03:28:27PM +0800, Mingyu He wrote: > The smaller the buffer_size in the test program, the more the > performance dropped. Initially, I used a 4k buffer_size, and the > performance decreased significantly. When the buffer_size was > increased to 128K, the read performance with RWF_DONTCACHE actually > surpassed the non-flagged version by about 10%. Maybe you have block size larger than 4k? Core-mm will allocate larger folios for page cache if filesystem asks it to. And if you try to access it with 4k buffer it gets multiple read-discard cycles for the same block with RWF_DONTCACHE. Without RWF_DONTCACHE only the first access to the block will lead to I/O, following accesses are served from page cache. -- Kiryl Shutsemau / Kirill A. Shutemov