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 X-Spam-Level: X-Spam-Status: No, score=-15.9 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1, USER_IN_DEF_DKIM_WL autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6FD03C433E0 for ; Sun, 24 May 2020 18:50:18 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 12C3F207DA for ; Sun, 24 May 2020 18:50:17 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="luQNcrLv" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 12C3F207DA Authentication-Results: mail.kernel.org; dmarc=fail (p=reject dis=none) header.from=google.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 5BED980008; Sun, 24 May 2020 14:50:17 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 56FE180007; Sun, 24 May 2020 14:50:17 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 45CDA80008; Sun, 24 May 2020 14:50:17 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0105.hostedemail.com [216.40.44.105]) by kanga.kvack.org (Postfix) with ESMTP id 2BC6580007 for ; Sun, 24 May 2020 14:50:17 -0400 (EDT) Received: from smtpin04.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id E14CE181AEF00 for ; Sun, 24 May 2020 18:50:16 +0000 (UTC) X-FDA: 76852502832.04.cause29_1f86e39a8de3f X-HE-Tag: cause29_1f86e39a8de3f X-Filterd-Recvd-Size: 7199 Received: from mail-oo1-f68.google.com (mail-oo1-f68.google.com [209.85.161.68]) by imf43.hostedemail.com (Postfix) with ESMTP for ; Sun, 24 May 2020 18:50:16 +0000 (UTC) Received: by mail-oo1-f68.google.com with SMTP id c194so1204346oob.6 for ; Sun, 24 May 2020 11:50:16 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:from:to:cc:subject:in-reply-to:message-id:references :user-agent:mime-version; bh=YzxjXCwPZcyq3dbQlFh0CZeB6bNa68G6QL/uQ90Wtp8=; b=luQNcrLvaWxW3N9dKSgbOzWHbVO2ID25t4Po1D700+18xa0egUySBpr/Qjimg6Gzol uI6+rtufd0qDmaC+vqQa9pA9KOtEBqwdL5rbZUa7ZEY7FLC3PV5Xy4+1ZL8+6Wr00KCa 337W3yBAsaPwGRDos7xkmhKWfo9VdJ0ofKK2scl9Vf1GAKo1pJsBnkc2qbb3zi983jVT ClrloMt6LI9DbVumUfwQ37QXALG/MNWv0v7nAtf2kUyr2iw39qNeu5Z4kpeIUbVQJkac 1O1O27M3ZQBxUy1j5Q6q650qJJCGzjP3aDpRxResDKirmpv+grqfn84eoEa8JM+V5rUb G1bQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:in-reply-to:message-id :references:user-agent:mime-version; bh=YzxjXCwPZcyq3dbQlFh0CZeB6bNa68G6QL/uQ90Wtp8=; b=mA7b9PCpexTjfO5vNafr3iqtTMNQe7F2Igp/24eOcCIDnxb+ZlzkZ7yzose0XM4Za1 /ND3qPJZ0hLqkI6Js02cXcT36kbEmVyyRYU8HcxEDBMe7fWUE+JS9lxhE6hkWpbtJjq0 p2Vrtlpo9QHXxFGAC282ohmQN9BdWq4LeNcthYZ92UAlJfGxTxXA91uyAtYpE2mW2thy 7267v1I+RRzFuDfLn8H/npjm+48nSoinWmXbg6uQTGDGi9mUHOBakYJopzf6XCzjEM8H xkD6w+ImkxIz2tVXKnKMmBoaAzu2VZtBtiu8oDwrLYPmxUtjmRhLYa9C2jGidEhzvyhk Qj9Q== X-Gm-Message-State: AOAM532LbfpOHy5Q1NPdUeRAOM4T/yLayIePUeJLgQZC6bKFhi+okAHL BsWIYYiFg8fDum334SBqd2RdEw== X-Google-Smtp-Source: ABdhPJxT8E3fKaVcGdfRYrxuKww4fxF9QYBsJduMqpCdojg6lDOKvPMuW3BAJ9XSptcthMaZ80lBPg== X-Received: by 2002:a4a:2809:: with SMTP id h9mr10815106ooa.36.1590346215532; Sun, 24 May 2020 11:50:15 -0700 (PDT) Received: from eggly.attlocal.net (172-10-233-147.lightspeed.sntcca.sbcglobal.net. [172.10.233.147]) by smtp.gmail.com with ESMTPSA id 95sm4462886otf.72.2020.05.24.11.50.13 (version=TLS1 cipher=ECDHE-ECDSA-AES128-SHA bits=128/128); Sun, 24 May 2020 11:50:14 -0700 (PDT) Date: Sun, 24 May 2020 11:49:58 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Konstantin Khlebnikov cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Hugh Dickins , Vlastimil Babka , David Rientjes , "Kirill A. Shutemov" Subject: Re: [PATCH v2] mm: remove VM_BUG_ON(PageSlab()) from page_mapcount() In-Reply-To: <159032779896.957378.7852761411265662220.stgit@buzz> Message-ID: References: <159032779896.957378.7852761411265662220.stgit@buzz> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII 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: On Sun, 24 May 2020, Konstantin Khlebnikov wrote: > Replace superfluous VM_BUG_ON() with comment about correct usage. > > Technically reverts commit 1d148e218a0d0566b1c06f2f45f1436d53b049b2 > ("mm: add VM_BUG_ON_PAGE() to page_mapcount()"), but context have changed. > > Function isolate_migratepages_block() runs some checks out of lru_lock > when choose pages for migration. After checking PageLRU() it checks extra > page references by comparing page_count() and page_mapcount(). Between > these two checks page could be removed from lru, freed and taken by slab. > > As a result this race triggers VM_BUG_ON(PageSlab()) in page_mapcount(). > Race window is tiny. For certain workload this happens around once a year. > > > page:ffffea0105ca9380 count:1 mapcount:0 mapping:ffff88ff7712c180 index:0x0 compound_mapcount: 0 > flags: 0x500000000008100(slab|head) > raw: 0500000000008100 dead000000000100 dead000000000200 ffff88ff7712c180 > raw: 0000000000000000 0000000080200020 00000001ffffffff 0000000000000000 > page dumped because: VM_BUG_ON_PAGE(PageSlab(page)) > ------------[ cut here ]------------ > kernel BUG at ./include/linux/mm.h:628! > invalid opcode: 0000 [#1] SMP NOPTI > CPU: 77 PID: 504 Comm: kcompactd1 Tainted: G W 4.19.109-27 #1 > Hardware name: Yandex T175-N41-Y3N/MY81-EX0-Y3N, BIOS R05 06/20/2019 > RIP: 0010:isolate_migratepages_block+0x986/0x9b0 > > > Code in isolate_migratepages_block() was added in commit 119d6d59dcc0 > ("mm, compaction: avoid isolating pinned pages") before adding VM_BUG_ON > into page_mapcount(). > > This race has been predicted in 2015 by Vlastimil Babka (see link below). Ah, kudos to Vlastimil. > > Signed-off-by: Konstantin Khlebnikov Thanks, Acked-by: Hugh Dickins For the BUG deletion. I got a bit confused by the comments at first: slight rewording suggested below. I had also intended a comment in page_mapcount(), on being used in that racy exceptional way by isolate_migratepages_block(): but now that the BUG is gone, I think you're right not to add that - it's not for the function to document its various callers, nor what's not in it. That kind of info belongs to the commit message, as you have done. > Fixes: 1d148e218a0d ("mm: add VM_BUG_ON_PAGE() to page_mapcount()") > Link: https://lore.kernel.org/lkml/557710E1.6060103@suse.cz/ > Link: https://lore.kernel.org/linux-mm/158937872515.474360.5066096871639561424.stgit@buzz/T/ (v1) > --- > include/linux/mm.h | 14 ++++++++++++-- > 1 file changed, 12 insertions(+), 2 deletions(-) > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 5a323422d783..95f777f482ac 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -782,6 +782,11 @@ static inline void *kvcalloc(size_t n, size_t size, gfp_t flags) > > extern void kvfree(const void *addr); > > +/* > + * Mapcount of compound page as a whole, not includes mapped sub-pages. s/not includes/does not include/ > + * > + * Must be called only for compound pages or any their tail sub-pages. > + */ > static inline int compound_mapcount(struct page *page) > { > VM_BUG_ON_PAGE(!PageCompound(page), page); > @@ -801,10 +806,15 @@ static inline void page_mapcount_reset(struct page *page) > > int __page_mapcount(struct page *page); > > +/* > + * Mapcount of 0-order page, for sub-page includes compound_mapcount(). s/, for sub-page/; when compound sub-page,/ > + * > + * Result is undefined for pages which cannot be mapped into userspace. > + * For example SLAB or special types of pages. See function page_has_type(). > + * They use this place in struct page differently. > + */ > static inline int page_mapcount(struct page *page) > { > - VM_BUG_ON_PAGE(PageSlab(page), page); > - > if (unlikely(PageCompound(page))) > return __page_mapcount(page); > return atomic_read(&page->_mapcount) + 1;