From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id A575EC48260 for ; Fri, 16 Feb 2024 16:17:09 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 2EE548D0007; Fri, 16 Feb 2024 11:17:09 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 29DE88D0002; Fri, 16 Feb 2024 11:17:09 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 13E998D0007; Fri, 16 Feb 2024 11:17:09 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id 04A2F8D0002 for ; Fri, 16 Feb 2024 11:17:09 -0500 (EST) Received: from smtpin11.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay03.hostedemail.com (Postfix) with ESMTP id 7D985A01CA for ; Fri, 16 Feb 2024 16:17:08 +0000 (UTC) X-FDA: 81798171336.11.671D68D Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by imf07.hostedemail.com (Postfix) with ESMTP id 5C51240012 for ; Fri, 16 Feb 2024 16:17:06 +0000 (UTC) Authentication-Results: imf07.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=THHLRs8N; dmarc=pass (policy=none) header.from=redhat.com; spf=pass (imf07.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1708100226; h=from:from:sender: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:dkim-signature; bh=c6+7atygW7QOnkGJLSuMLDIxvn9DIzwdT1QSyiTLJNA=; b=AfmYX2bxgMips/vi+lWDZlslmet2RX4tK6ZFFZlcPYdbvSreLGZTkXZwBUoQIdXqdTQdWc nTM9iptK1h3juDWQ63BwWUlmXVNxR6kIuVc8OJ0iMVVteKEJesmVdEgQVxPfOZSExzAE2N wFv9/5/TwtdaFug6RctzcZzFyXfK84c= ARC-Authentication-Results: i=1; imf07.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=THHLRs8N; dmarc=pass (policy=none) header.from=redhat.com; spf=pass (imf07.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1708100226; a=rsa-sha256; cv=none; b=1egUlhTpiWVAjX9RjrHRJteflf3uGsjZ1bTiP3nLJ5eob9UubIeq9zb7ll9bTV3OZHOP+8 Ny0ybwuhdqwpQyWV3lltw++Glkn7oorxP8rYtBpT+BUMVu6gxDbhDagJg1qS44FGy4T1hG 8fQaRDey5exBdqtqxTUQL1lYTBAlemo= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1708100225; 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=c6+7atygW7QOnkGJLSuMLDIxvn9DIzwdT1QSyiTLJNA=; b=THHLRs8NcYzyKlEFk8vjjPu9AYE7hwphwlrOq3F3snVvWK3Z/2Mq7w3CU2Q4j0ApoO7fQC ab5oS3fEveHRnGS6ETwzOP2pYlFwmjA8Smc1VkmpVeU1bdHOpI0/hIPEuh43M/UuS0Os8L 2x8E91YzI//3qZMJYtE4tc1YaWh5hOI= Received: from mail-wm1-f71.google.com (mail-wm1-f71.google.com [209.85.128.71]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-107-R32agw9BPXyqWv9wJ0FN0Q-1; Fri, 16 Feb 2024 11:17:04 -0500 X-MC-Unique: R32agw9BPXyqWv9wJ0FN0Q-1 Received: by mail-wm1-f71.google.com with SMTP id 5b1f17b1804b1-4120a19da9dso11506235e9.3 for ; Fri, 16 Feb 2024 08:17:03 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1708100223; x=1708705023; h=content-transfer-encoding:in-reply-to:organization:autocrypt:from :content-language: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=c6+7atygW7QOnkGJLSuMLDIxvn9DIzwdT1QSyiTLJNA=; b=XSIpC3aEmeOZfMukUrytlNrgxpusjpeSnZrngNRghUSNeiShjxTJcfmbSg8DJNu+pX XfXkx+lsz+Gu36sZiAUX6n4gO29Wjem9S45qOwmvevsUsVOEIZ0m0qMK1UrKEwbH043o Mvl7WRRHFpJqmavHwWDxvlJBiST9dstWmSL+ENCi7RItwjXTs+b1+51fLaD2nxakXfow MG8jRhVw9s2L5tM9Fq6B+S/l99tY77XLJvQBb3lXAm0PGPJQpQTqiDw6S1TlOSCQU5IR RQ5iuw0rA4NTjuhrvaAhs5hon6sm6qw318ud+dCOHopGUI0AqNb/FMKsXMebwD4ruOBf 4rEg== X-Gm-Message-State: AOJu0YyogkVu+IQ5yUHk3BlKt4hGbvpPkWrBvzzeIaK7YO7Z6twCjUJ1 /Ljwvwbx3101k8aEsnXOxlwJNHCXXYrVtOEU5sw22LGhdN+Tl6M4SViD5OqIZ4XX+KDSwn19pag FlGWlPkwVnKO25LLaAUFCCBCqQkxv7kYVqW3feD6eseB4Po6Z X-Received: by 2002:a05:600c:1c1e:b0:412:1615:7343 with SMTP id j30-20020a05600c1c1e00b0041216157343mr4367159wms.5.1708100222924; Fri, 16 Feb 2024 08:17:02 -0800 (PST) X-Google-Smtp-Source: AGHT+IFY2ktOfjymj6X1+czeKaqGXtMp53zw558F+DnAgKuEALM2JWjcOjKlnalz2uom4G6GaaT7Tw== X-Received: by 2002:a05:600c:1c1e:b0:412:1615:7343 with SMTP id j30-20020a05600c1c1e00b0041216157343mr4366088wms.5.1708100192759; Fri, 16 Feb 2024 08:16:32 -0800 (PST) Received: from ?IPV6:2003:d8:2f3c:3f00:7177:eb0c:d3d2:4b0e? (p200300d82f3c3f007177eb0cd3d24b0e.dip0.t-ipconnect.de. [2003:d8:2f3c:3f00:7177:eb0c:d3d2:4b0e]) by smtp.gmail.com with ESMTPSA id e25-20020a05600c219900b0040fe4b733f4sm2719927wme.26.2024.02.16.08.16.31 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 16 Feb 2024 08:16:32 -0800 (PST) Message-ID: <6ad329f6-6e1b-411b-a5d7-be1c8fd89d96@redhat.com> Date: Fri, 16 Feb 2024 17:16:31 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/swap: fix race when skipping swapcache To: Kairui Song Cc: linux-mm@kvack.org, Andrew Morton , "Huang, Ying" , Chris Li , Minchan Kim , Yu Zhao , Barry Song , SeongJae Park , Hugh Dickins , Johannes Weiner , Matthew Wilcox , Michal Hocko , Yosry Ahmed , stable@vger.kernel.org, linux-kernel@vger.kernel.org References: <20240206182559.32264-1-ryncsn@gmail.com> <1d259a51-46e6-4d3b-9455-38dbcc17b168@redhat.com> <4c651673-132f-4cd8-997e-175f586fd2e6@redhat.com> 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 ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: dmzzoj8b9gpoifmn8j9orgb9au137iid X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 5C51240012 X-HE-Tag: 1708100226-313022 X-HE-Meta: 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 BnMZ6Rj6 xszrRye3AAwtvZgleuXUjXWgukNop1il1GJpaGEoUc1k79FL9RyB/ouqGePHwzQk4LLVOS5QLGMYXbuG6su3TBMBF+f3X41yGUn83tDjxV8TtMSL1L6V427DSatXXPQMe9LX8BJLAETgLtaBnwuGfBhkt/3xI77RcvnGDBU4tiUUpS/U= 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: >> (4) relock the folio. (we do that already, might not want to fail) >> >> (4) take the PTE lock. If the PTE did not change, turn it into a present >> PTE entry. Otherwise, cleanup. > > Very interesting idea! > > I'm just not sure what actual benefit it brings. The only concern > about reusing swapcache_prepare so far is repeated page faults that > may hurt performance or statistics, this issue is basically gone after > adding a schedule(). I think you know that slapping in random schedule() calls is not a proper way to wait for an event to happen :) It's pretty much unpredictable how long the schedule() will take and if there even is anything to schedule to! With what I propose, just like with page migration, you really do wait for the event (page read and unlocked, only the PTE has to be updated) to happen before you try anything else. Now, the difference is most likely that the case here happens much less frequently than page migration. Still, you could have all threads faulting one the same page and all would do the same dance here. > > We can't drop all the operations around swap cache and map anyway. It > doesn't know if it should skip the swapcache until swapcache lookup > and swap count look up are all done. So I think it can be done more > naturally here with a special value, making things simpler, robust, > and improving performance a bit more. > The issue will always be that someone can zap the PTE concurrently, which would free up the swap cache. With what I propose, that cannot happen in the sync swapin case we are discussing here. If someone where to zap the PTE in the meantime, it would only remove the special non-swap entry, indicating to swapin code that concurrent zapping happened. The swapin code would handle the swapcache freeing instead, after finishing reading the content. So the swapcache would not get freed concurrently anymore if I am not wrong. At least the theory, I didn't prototype it yet. > And in another series [1] I'm working on making shmem make use of > cache bypassed swapin as well, following this approach I'll have to > implement another shmem map based synchronization too. > I'd have to look further into that, if something like that could similarly apply to shmem. But yes, it's no using PTEs, so a PTE-based sync mechanism does definitely not apply.. > After all it's only a rare race, I think a simpler solution might be better. I'm not sure that simpler means better here. Simpler might be better for a backport, though. The "use schedule() to wait" looks odd, maybe it's a common primitive that I simply didn't stumble over yet. (I doubt it but it could be possible) -- Cheers, David / dhildenb