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[209.85.208.52]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-b48652aa62csm1455262666b.19.2025.10.07.15.35.44 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 07 Oct 2025 15:35:45 -0700 (PDT) Received: by mail-ed1-f52.google.com with SMTP id 4fb4d7f45d1cf-6394938e0ecso9134951a12.1 for ; Tue, 07 Oct 2025 15:35:44 -0700 (PDT) X-Forwarded-Encrypted: i=1; AJvYcCWxoYgkYyzgKGF9+mr1x/TsV25cRoRtCmAC4zRWRsU9Bo9pGnrc/5ARcBH30S1wmw3Gurkvqopo3w==@kvack.org X-Received: by 2002:a05:6402:26c7:b0:639:dab5:d610 with SMTP id 4fb4d7f45d1cf-639dab5dab3mr251304a12.15.1759876544657; Tue, 07 Oct 2025 15:35:44 -0700 (PDT) MIME-Version: 1.0 References: <4bjh23pk56gtnhutt4i46magq74zx3nlkuo4ym2tkn54rv4gjl@rhxb6t6ncewp> <5zq4qlllkr7zlif3dohwuraa7rukykkuu6khifumnwoltcijfc@po27djfyqbka> In-Reply-To: From: Linus Torvalds Date: Tue, 7 Oct 2025 15:35:27 -0700 X-Gmail-Original-Message-ID: X-Gm-Features: AS18NWD-EYOWOuMnn_EcBjH4dRmdOnLeMLRD6pujUfuQrjY9wz8yiIq4-85QYEU Message-ID: Subject: Re: Optimizing small reads To: Kiryl Shutsemau Cc: Matthew Wilcox , Luis Chamberlain , Linux-MM , linux-fsdevel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam05 X-Stat-Signature: wcqaz5kfcstkz6yhjqjnxm5bks6c9ebf X-Rspam-User: X-Rspamd-Queue-Id: 70A17100010 X-HE-Tag: 1759876547-136317 X-HE-Meta: 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 2YpE/v5F 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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: On Tue, 7 Oct 2025 at 14:47, Linus Torvalds wrote: > > Note: this also ends up doing it all with pagefaults disabled, becasue > that way we can do the looping without dropping and re-taking the RCU > lock. I'm not sure that is sensible, but whatever. Considering that I > didn't test this AT ALL, please just consider this a "wild handwaving" > patch. It might be worth noting that the real reason for disabling page faults is that I think the whole loop should be moved into filemap_fast_read(), and then the sequence count read - with the memory barrier - would be done only once. Plus at that point the code can do the page lookup only once too. So that patch is not only untested, it's also just a "step one". But I think I'll try to boot it next. Wish me luck. Linus