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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2D884C433F5 for ; Tue, 15 Mar 2022 15:45:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1349701AbiCOPqb (ORCPT ); Tue, 15 Mar 2022 11:46:31 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56792 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233153AbiCOPqa (ORCPT ); Tue, 15 Mar 2022 11:46:30 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 8494013D6D for ; Tue, 15 Mar 2022 08:45:18 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1647359117; 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; bh=bYl1LSBqhgpJOo7T6+g24bgPe/Xpp9f9wR/UlrFgmkY=; b=bEimwjB7kvETAn4XMfFwiJe0hZNoakIphJPVkHcRW+9p6sUdJncxyQff4sa0HOi1T4nrH2 ub9OJ/UdwYfYqFd/fGhZaHFoqg5i0GOM3+lMZ05Ls5/UrQM10rc5bqJQR/qBgCAVv/M0g9 DeQKoAqTaRzFZFVSFjY+OR+PDUA31yY= Received: from mail-wr1-f69.google.com (mail-wr1-f69.google.com [209.85.221.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-386-d44pE-LWNOGMALu-vjHiuQ-1; Tue, 15 Mar 2022 11:45:16 -0400 X-MC-Unique: d44pE-LWNOGMALu-vjHiuQ-1 Received: by mail-wr1-f69.google.com with SMTP id t8-20020adfa2c8000000b001e8f6889404so5418362wra.0 for ; Tue, 15 Mar 2022 08:45:15 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:message-id:date:mime-version:user-agent:subject :content-language:to:cc:references:from:organization:in-reply-to :content-transfer-encoding; bh=bYl1LSBqhgpJOo7T6+g24bgPe/Xpp9f9wR/UlrFgmkY=; b=EZrkTxOpkHuxNNSdjSOhN9S1GHvlHqEWOB3BcQcwK337V/SCSfRX0tzfRAa1z/CVuN 95HCXvRamXx5djwYCumwNP3KcPxv5BxmGkSpASdWKfytBbW0wScEY+eZ4eswmoH/1C6T 88Au3AWooY0hMvLe99hiVegNPVn+qjxN9xujOcJyFt3HiRN4+pPrVPxwDZv9o50VobOa LfKRZFDP/Ey0JSRGyEHMVskFDXvhIFaL6iLQaRDR5wEbPUbP68SOlnn/sTyx+z8lYnQv jH+CIK7lscZlwPLhxx8KRbQ5RpAwDa47sRXKwseWu5tbcTRJC7kBGQ7HJPDBslyGQPns DQvw== X-Gm-Message-State: AOAM5317FNuxq4WpKzbTnELlW6hcIaHfU+UeLqQRUyGKMDqJD0SBS/Pa xr+PUo0gR8zaSjPEin0M8ROO2JCk9YCc3fAtBH5Vuxu+WCoi7UuKFvBoPtqZPBY0xb+ZonQ2eMO UNhmofXaJG0ZZ/z3Z0+dURPf4 X-Received: by 2002:a5d:5887:0:b0:1f1:eb44:724e with SMTP id n7-20020a5d5887000000b001f1eb44724emr21315063wrf.44.1647359114942; Tue, 15 Mar 2022 08:45:14 -0700 (PDT) X-Google-Smtp-Source: ABdhPJzPoEJ1GBznl0gSlJSA8PP9PMIE7udTKKIyZAmfT4CMnFBi93qUV9w8thhZ2jRKEb3NPxfnVA== X-Received: by 2002:a5d:5887:0:b0:1f1:eb44:724e with SMTP id n7-20020a5d5887000000b001f1eb44724emr21315036wrf.44.1647359114709; Tue, 15 Mar 2022 08:45:14 -0700 (PDT) Received: from ?IPV6:2003:cb:c708:1800:42bd:3cac:d22a:3c62? (p200300cbc708180042bd3cacd22a3c62.dip0.t-ipconnect.de. [2003:cb:c708:1800:42bd:3cac:d22a:3c62]) by smtp.gmail.com with ESMTPSA id r4-20020a1c2b04000000b0038a0e15ee13sm2637525wmr.8.2022.03.15.08.45.13 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 15 Mar 2022 08:45:14 -0700 (PDT) Message-ID: <4cb789a5-c49c-f095-1f7e-67be65ba508a@redhat.com> Date: Tue, 15 Mar 2022 16:45:13 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.6.2 Subject: Re: [PATCH] mm/migrate: fix race between lock page and clear PG_Isolated Content-Language: en-US To: Andrew Morton , Andrew Yang Cc: Matthias Brugger , Matthew Wilcox , Vlastimil Babka , David Howells , William Kucharski , Yang Shi , Marc Zyngier , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-mediatek@lists.infradead.org, wsd_upstream@mediatek.com, Nicholas Tang , Kuan-Ying Lee References: <20220315030515.20263-1-andrew.yang@mediatek.com> <20220314212127.a2797926ee0ef8a7ad05dcaa@linux-foundation.org> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20220314212127.a2797926ee0ef8a7ad05dcaa@linux-foundation.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 15.03.22 05:21, Andrew Morton wrote: > On Tue, 15 Mar 2022 11:05:15 +0800 Andrew Yang wrote: > >> When memory is tight, system may start to compact memory for large >> continuous memory demands. If one process tries to lock a memory page >> that is being locked and isolated for compaction, it may wait a long time >> or even forever. This is because compaction will perform non-atomic >> PG_Isolated clear while holding page lock, this may overwrite PG_waiters >> set by the process that can't obtain the page lock and add itself to the >> waiting queue to wait for the lock to be unlocked. >> >> CPU1 CPU2 >> lock_page(page); (successful) >> lock_page(); (failed) >> __ClearPageIsolated(page); SetPageWaiters(page) (may be overwritten) >> unlock_page(page); >> >> The solution is to not perform non-atomic operation on page flags while >> holding page lock. > > Sure, the non-atomic bitop optimization is really risky and I suspect > we reach for it too often. Or at least without really clearly > demonstrating that it is safe, and documenting our assumptions. I agree. IIRC, non-atomic variants are mostly only safe while the refcount is 0. Everything else is just absolutely fragile. -- Thanks, David / dhildenb