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[2003:d8:2f3f:4b00:ee13:8c22:5cc5:d169]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-475dd4cc596sm147684725e9.15.2025.10.27.09.06.34 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 27 Oct 2025 09:06:35 -0700 (PDT) Message-ID: <7bfd0822-5687-4ddc-9637-0cedd404c34e@redhat.com> Date: Mon, 27 Oct 2025 17:06:34 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/filemap: Implement fast short reads To: Linus Torvalds , Hugh Dickins Cc: Kiryl Shutsemau , Andrew Morton , Matthew Wilcox , Alexander Viro , Christian Brauner , Jan Kara , Yang Shi , Dave Chinner , Suren Baghdasaryan , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org References: <20251017141536.577466-1-kirill@shutemov.name> From: David Hildenbrand Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZoEEwEIAEQCGwMCF4ACGQEFCwkIBwICIgIG FQoJCAsCBBYCAwECHgcWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaJzangUJJlgIpAAKCRBN 3hD3AP+DWhAxD/9wcL0A+2rtaAmutaKTfxhTP0b4AAp1r/eLxjrbfbCCmh4pqzBhmSX/4z11 opn2KqcOsueRF1t2ENLOWzQu3Roiny2HOU7DajqB4dm1BVMaXQya5ae2ghzlJN9SIoopTWlR 0Af3hPj5E2PYvQhlcqeoehKlBo9rROJv/rjmr2x0yOM8qeTroH/ZzNlCtJ56AsE6Tvl+r7cW 3x7/Jq5WvWeudKrhFh7/yQ7eRvHCjd9bBrZTlgAfiHmX9AnCCPRPpNGNedV9Yty2Jnxhfmbv Pw37LA/jef8zlCDyUh2KCU1xVEOWqg15o1RtTyGV1nXV2O/mfuQJud5vIgzBvHhypc3p6VZJ lEf8YmT+Ol5P7SfCs5/uGdWUYQEMqOlg6w9R4Pe8d+mk8KGvfE9/zTwGg0nRgKqlQXrWRERv cuEwQbridlPAoQHrFWtwpgYMXx2TaZ3sihcIPo9uU5eBs0rf4mOERY75SK+Ekayv2ucTfjxr Kf014py2aoRJHuvy85ee/zIyLmve5hngZTTe3Wg3TInT9UTFzTPhItam6dZ1xqdTGHZYGU0O otRHcwLGt470grdiob6PfVTXoHlBvkWRadMhSuG4RORCDpq89vu5QralFNIf3EysNohoFy2A LYg2/D53xbU/aa4DDzBb5b1Rkg/udO1gZocVQWrDh6I2K3+cCs7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: JK_agADzs6xP8iat09mdkQhttiKfHtUBAmHvOl-DJyw_1761581196 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam01 X-Stat-Signature: geu7c8o8q83xeb7znyhqaixyxntq36fk X-Rspam-User: X-Rspamd-Queue-Id: 7792B40018 X-HE-Tag: 1761581199-807710 X-HE-Meta: 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 NX4B1zUh LWFALBA9Uemt7jc7jMJ65GjvSuB/2q2aKtYN9ywzgRPKosfwpHNB7U57nFIeTIUGEdbhhQzSm75y9g1ew5zGL1THhRwy+WJodbXULnRqJqVxN3euSG52BcGK9UfJgs9TqhEGDxMNHtgJ7aM0BQ4IoxsYN3LcEPgy1EWNvLJu+AxJjlymxDpkQjOSzZRNOxCZD5svo+MONQPsf/phMKoxSW58QNOik8vYGw8lTX9dW0wiC9xYSp6KgTRHaNY8RIcfj11XcIiUDB1aDplRMcDaL0Ja47sO+JKeYUJ/8WQn99qIWo6m2lMRzgNZ3O4wvw5IgaMvOMcfJEItQo8rq3JdOVKLn/aDdrNjxJvfIsmBCqqeWIZpL+++iYhdqfkJw1aDdDUs5zVaV/iUOAu8XV7HUnD82C7T37R4qb/9n 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 27.10.25 16:50, Linus Torvalds wrote: > On Mon, 27 Oct 2025 at 03:49, Hugh Dickins wrote: >> >> This makes a fundamental change to speculative page cache assumptions. > > Yes, but I'm a bit surprised by people who find that scary. > > The page cache does *much* more scary things elsewhere, particularly > the whole folio_try_get() dance (in filemap_get_entry() and other > places). > > I suspect you ignore that just because it's been that way forever, so > you're comfortable with it. > > I'd argue that that is much *much* more subtle because it means that > somebody may be incrementing the page count of a page that has already > been re-allocated by somebody else. > > Talk about cognitive load: that code makes you think that "hey, the > tryget means that if it has been released, we don't get a ref to it", > because that's how many of our *other* speculative RCU accesses do in > fact work. > > But that's not how the page cache works, exactly because freeing isn't > actually RCU-delayed. > > So while the code visually follows the exact same pattern as some > other "look up speculatively under RCU, skip if it's not there any > more", it actually does exactly the same thing as the "copy data under > RCU, then check later if it was ok". Except it does "increment > refcount under RCU, then check later if it was actually valid". > > That said, I wonder if we might not consider making page cache freeing > be RCU-delayed. This has come up before (exactly *because* of that > "folio_try_get()"). > > Because while I am pretty sure that filemap_get_entry() is fine (and a > number of other core users), I'm not convinced that some of the other > users of folio_try_get() are necessarily aware of just how subtle that > thing is. > > Anyway, I'm certainly not going to push that patch very hard. I will sleep better at night if we can guarantee that we are reading from a folio that has not been reused in the meantime -- or reading random other memory as I raised in my other mail. So I really wish that we can defer optimizing this to freeing folios under RCU instead. -- Cheers David / dhildenb