From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E2FAC30DD2A for ; Mon, 27 Oct 2025 16:06:39 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761581201; cv=none; b=pEP3AKKU90qeqCw2C6kMVwiOFdlvkyYmnkxcyus4y7jpoMuE0rgBVnPg+PG60H7Cbg5AayswhqVjMYm7kzwtNeicyCvirWhJgpmufKwAIxUQme5kO70STTrEPzsDQb1POGTbFDAOhvPs1AJVVxmOIy42eAs97HijXDTwLFQrOh8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761581201; c=relaxed/simple; bh=QaWPD/XxXcWJkRndKn0ialPXtuOPWjCwlbSKvLXVWL0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=CV55LoQ4XEOC+5OucDoOylk3HEL5cn1GLG/RmeUoKpXFUD8PEQOp4rGdSAbtFTz7gPVw8VpM3yvsG4fNCtZmPVc847dM9Vz8+EMEJbGizva7W0/DrH7Iyaf+kRple4rvAgWy4sMeJLbnJd0mwf2+BryHedNpMPdp3j0fSIjfFg4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=HA/0OXMd; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="HA/0OXMd" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1761581198; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:autocrypt:autocrypt; bh=kpxdS5ygvVp4sssexZhDityvniMqB9yz0acKtxZ1LBk=; b=HA/0OXMdlw29QT44TH66HnEKUjp/yuTvp+Kij7dAeYc7jn9RcB7hhHFReoQqgUWwhaTVBd 6W/G7+9riiSuFR7QN5UorADLDNbTJiZd1fxnZyacutyqexIAovVG0PGf2A1NwMc0H/dSj0 x9yJDgpm+QHyQ+LsOhQbMlrM4pXt++4= Received: from mail-wm1-f69.google.com (mail-wm1-f69.google.com [209.85.128.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-122-9jZO8z2TOWK8s-9nFvgkLg-1; Mon, 27 Oct 2025 12:06:37 -0400 X-MC-Unique: 9jZO8z2TOWK8s-9nFvgkLg-1 X-Mimecast-MFC-AGG-ID: 9jZO8z2TOWK8s-9nFvgkLg_1761581196 Received: by mail-wm1-f69.google.com with SMTP id 5b1f17b1804b1-475da25c4c4so11743085e9.0 for ; Mon, 27 Oct 2025 09:06:37 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1761581196; x=1762185996; h=content-transfer-encoding:in-reply-to:autocrypt:content-language :from:references:cc:to:subject:user-agent:mime-version:date :message-id:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=kpxdS5ygvVp4sssexZhDityvniMqB9yz0acKtxZ1LBk=; b=CvBJy5Yfc3ox7IYsnmPOB3wJNF4IKU+NvTzVgjU2S3QeqQF6vExSl55VNo/+yqsDmZ Rw2K4o+UZH51mcyUvU+n8sseJJovyD7ih51YHgABZTSjwDQw+VyCLfWc+xxtU85BW+e/ cea07T2EeyKemeOcddXra/phT0inAJdcMN0G0rt+CFjOKZKH7I9K+SdcxxCRKQjSsIL3 +pKXVROkydpjQFSZxf6d219foNvx1I9bTfxHprPrmoiHoS2i2l9Z6v4wu4R+tjwNVKTj ITWk84PPnb1BO8jDBHk3xDinAcYC5HkIA0/NbeJEuNsckgfCx9eu3QaTbwyZeZzxhZXh 26BQ== X-Forwarded-Encrypted: i=1; AJvYcCUAi5w12uXBRjKijGxFz745ENxUoS2FP4240S9fx0HXJkMFZVnIixgO+F5jojuZOn/zZvsLQUGSWaDJaO2i@vger.kernel.org X-Gm-Message-State: AOJu0YziI3TlI8zOsekHTNW8w67T6F9PFsaAFAXUN6wqfDKrLSqB888n sd7BWQ9M86ssFKsMO13Mm6vHrYXlk7u90TKXF/yiw9cK/Mx2qmYm8uZZkdLzDzHmlW0Rvx3RJ83 G9Iu8AJjbI5NGwqWFdzrsBe2YUrakO2yLutOaHwyUC0AW6ohlWLY4JoVB4+2Sl+Wp77U= X-Gm-Gg: ASbGncvx6k+SCYHrpkJCry5NdxZZhLxQmTVq8aidpsBdM3AEqY3E1mA07U2/SjYoRuh DDsBFdtjmMQK3VQa4PvZprKbWxUPCBXj59++ZosOIGvzK/wCIogmiDD/gulZpqd51OF18t1i8BE RPPw7+v8nBh/3WHbldCo48W7Ol62LQtjd4wax3r1bNNUOtudNGvY2unUDxohgAfVe8tBdgdSnsn X8E7vzzgq84mStIDWzHXy7cqIhnph7TJeHKKvLJgEZhQQfqOVwUl9xjvW1z7J0ZX8ri9xXqlCAt 8zj9xyl4vMWSRYj6dKOEox6F27qqzwvCiQfE1xHluzHmOHMkf7o4fX8UGzdwSs7gKAnn16MrsK0 FY273XkMldwq83kJecF0q1vVFJ1BO1ZJMbngMfo9lxNcXKkQWuPtTO5yeDOH6H4MifqYGPblSOE DeAfWnHGpGj7+2OZPu2Nse72Ow0EE= X-Received: by 2002:a05:600c:4fc4:b0:46f:b327:ecfb with SMTP id 5b1f17b1804b1-47717e03722mr1896875e9.9.1761581196177; Mon, 27 Oct 2025 09:06:36 -0700 (PDT) X-Google-Smtp-Source: AGHT+IHv5M9b4q9OVKGnW+HmaEJg2ucSNwvJY0LUs9zHR8mcSnR+YxmFwPGnp36vYB+8STF/MebLhQ== X-Received: by 2002:a05:600c:4fc4:b0:46f:b327:ecfb with SMTP id 5b1f17b1804b1-47717e03722mr1896525e9.9.1761581195766; Mon, 27 Oct 2025 09:06:35 -0700 (PDT) Received: from ?IPV6:2003:d8:2f3f:4b00:ee13:8c22:5cc5:d169? (p200300d82f3f4b00ee138c225cc5d169.dip0.t-ipconnect.de. [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 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 Content-Language: en-US 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: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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