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[209.85.218.47]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-b4869c4f74esm467775666b.67.2025.10.03.09.19.10 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 03 Oct 2025 09:19:10 -0700 (PDT) Received: by mail-ej1-f47.google.com with SMTP id a640c23a62f3a-b3e25a4bfd5so485062166b.2 for ; Fri, 03 Oct 2025 09:19:10 -0700 (PDT) X-Forwarded-Encrypted: i=1; AJvYcCXFscwsaVG66no8fD+X0ZuemuvyQSzFnSo9P+9Y4u1AYZsaIeTctZWVqS+OTfVNA3rYYFZ9CpAE9g==@kvack.org X-Received: by 2002:a17:907:7286:b0:b3d:e9be:7ac2 with SMTP id a640c23a62f3a-b49c1278127mr425786066b.5.1759508349852; Fri, 03 Oct 2025 09:19:09 -0700 (PDT) MIME-Version: 1.0 References: In-Reply-To: From: Linus Torvalds Date: Fri, 3 Oct 2025 09:18:52 -0700 X-Gmail-Original-Message-ID: X-Gm-Features: AS18NWDV_Wr0ZuaC3UZb73XNzshbvF6xfGPRkb7B7SRlcIpX-t0bMDfrJbAe05w Message-ID: Subject: Re: Optimizing small reads To: Kiryl Shutsemau Cc: Matthew Wilcox , Luis Chamberlain , Linux-MM Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 3ABCE100011 X-Stat-Signature: 8bkjh7ub41km9fh8p73hfjtkzw6xcskz X-Rspam-User: X-HE-Tag: 1759508353-792672 X-HE-Meta: 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 as9A659z 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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 Fri, 3 Oct 2025 at 02:55, Kiryl Shutsemau wrote: > > So, you intentionally bypassing refcount bump on the folio which is part > of the usual page cache lookup protocol. Yes. The big cost that Willy reported originally was the cacheline bouncing of the refcount (due to having tons and tons of very small reads on the same page in a very threaded load). My initial reaction was that it was a completely unrealistic microbenchmark (who would read the same page from multiple threads at the same time unless they were just doing some silly benchmark with no real-world application?), but apparently there are real loads. > But without the pin, what prevents the folio from being freed and > reallocated in the same spot under you? Do we wait for a grace period > somewhere in this reallocation cycle? I don't see it. That was one of the theories and it might be the right one. There was some discussion about having a sequence number. It's a long time ago, I forget. I had some reason I thought it wasn't an issue, but after all this time I can't recall. There's a reason that patch was marked 'Not-yet-signed-off-by' :) Linus