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 72A8BC7EE29 for ; Fri, 9 Jun 2023 20:03:13 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 075CC8E0006; Fri, 9 Jun 2023 16:03:13 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 0257D8E0002; Fri, 9 Jun 2023 16:03:12 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id E09768E0006; Fri, 9 Jun 2023 16:03:12 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0010.hostedemail.com [216.40.44.10]) by kanga.kvack.org (Postfix) with ESMTP id CFEBE8E0002 for ; Fri, 9 Jun 2023 16:03:12 -0400 (EDT) Received: from smtpin23.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay05.hostedemail.com (Postfix) with ESMTP id A2419402DA for ; Fri, 9 Jun 2023 20:03:12 +0000 (UTC) X-FDA: 80884283424.23.3D8B876 Received: from mail-oi1-f170.google.com (mail-oi1-f170.google.com [209.85.167.170]) by imf28.hostedemail.com (Postfix) with ESMTP id 7F5D3C000E for ; Fri, 9 Jun 2023 20:03:10 +0000 (UTC) Authentication-Results: imf28.hostedemail.com; dkim=pass header.d=google.com header.s=20221208 header.b=pKopmHpv; spf=pass (imf28.hostedemail.com: domain of hughd@google.com designates 209.85.167.170 as permitted sender) smtp.mailfrom=hughd@google.com; dmarc=pass (policy=reject) header.from=google.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1686340990; 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: in-reply-to:in-reply-to:references:references:dkim-signature; bh=LRHbZHM11tMZIbHCANoNbk0Dr/SrecuO1LYm99EHhK4=; b=k9riOSmMhvq7SfdBQIOeWAOsIZlVMYbLzDACTPjjeSG9oMEZzvvOjPZKQrdjNwWaMuXr45 SzvRh4YNf/MZsHsIO1szjETAGqYUT7yLuIajUN9n4tDbghZivPrKAldK/w05I1mfxv6Tc2 J2bgMO9laH5kSxFgPXmscve2SB/EdWA= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1686340990; a=rsa-sha256; cv=none; b=6fhYEpWEoFqm3QCPLbZTEzGaoLyadLKjsdfk+YXowTLvl+GpVPZOspcstm3VNzoFjea23J bpWMe0V6FnItlD/Q/FznN24tL1S3VFsx51XMte2sYItH1EIxhMI1iMRvVp3dP4PoIUS+4L VhC6Oq6WO1VwipnTdN9uSymBsB87Uzc= ARC-Authentication-Results: i=1; imf28.hostedemail.com; dkim=pass header.d=google.com header.s=20221208 header.b=pKopmHpv; spf=pass (imf28.hostedemail.com: domain of hughd@google.com designates 209.85.167.170 as permitted sender) smtp.mailfrom=hughd@google.com; dmarc=pass (policy=reject) header.from=google.com Received: by mail-oi1-f170.google.com with SMTP id 5614622812f47-39aa8a055e0so901497b6e.0 for ; Fri, 09 Jun 2023 13:03:10 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20221208; t=1686340989; x=1688932989; h=mime-version:references:message-id:in-reply-to:subject:cc:to:from :date:from:to:cc:subject:date:message-id:reply-to; bh=LRHbZHM11tMZIbHCANoNbk0Dr/SrecuO1LYm99EHhK4=; b=pKopmHpvLeR2joACRQv/ngHApkhi7F77iUQjLzlcpcRbo811qWJ5hAHeevhQvRLtyI HTwZMl88uQywWblTh/gCI7s3E48tvaIbifTvtg6ILE6YNjJAfl8M8zs+OOGhZSAos1Ox 9FXL4bozv1Nn+387JrzoA+UoxIaxmWCFkTCmZ+WpP8Mu0epo74fPgCfKZlgV0bMrXwZw ST2f4F3DrLK8DBfmJ1d7JDQYK/zo2Xl3avD9AbwgPyo6PqRIJAZdvgOuEsq52804TsDt P3MUcNRYchtpmATdOKOBQURRdfWZBasxJh1w14WF/4O8Htc+iec+juBDjLV2ClLcANlw fI3Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1686340989; x=1688932989; h=mime-version:references:message-id:in-reply-to:subject:cc:to:from :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=LRHbZHM11tMZIbHCANoNbk0Dr/SrecuO1LYm99EHhK4=; b=ZzUxe8LemT14iTrARxdWbOjNHnHiq+28mvfvV2SIK6krqgTBykSHW7C+TvR7KnszLU 2ZWEkpKqEY+xjLdyoYgCIi5BAREflnkOltYrJNXb20pkSoA4jaxWgyj3N7I8UvZFBPAK sVjPAztQ0GOKOYmAbcz+hrMTXxQuh0QAYT+6MiMMN0Uzs04hEM+s5tkhQ1OryQjQUZn5 f/bmnvIo9KStbpR9F4ChN79uVuvY59dNa4XrnWNjepf3Ovjo/MeNL+J/5h4F+Se5cOZF BUQW1JFIoWibefS3IVitXtTZ2DUZkYEar8uHMLGfzPZSmu+GPjiS7TZGNBtG4EWAdVRX eTzw== X-Gm-Message-State: AC+VfDysMJ/y1/uxwDxONqAdeHpQuqTdp6PQWNwFM56zIGmdLvdLP7nB J3A27btcmGImQfpzqNxOvaY5Cg== X-Google-Smtp-Source: ACHHUZ7ztmiTxP5Bz5ivt78H90m8jQbJXcKOudfas683UnOJYkelXLZWc/vYZCmwqAIDfFXZ6saujg== X-Received: by 2002:a05:6359:d1c:b0:123:3f86:fa66 with SMTP id gp28-20020a0563590d1c00b001233f86fa66mr2382047rwb.5.1686340989284; Fri, 09 Jun 2023 13:03:09 -0700 (PDT) Received: from ripple.attlocal.net (172-10-233-147.lightspeed.sntcca.sbcglobal.net. [172.10.233.147]) by smtp.gmail.com with ESMTPSA id t69-20020a0dea48000000b00568d63f4b46sm791921ywe.133.2023.06.09.13.03.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 09 Jun 2023 13:03:08 -0700 (PDT) Date: Fri, 9 Jun 2023 13:02:59 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@ripple.attlocal.net To: David Stevens cc: linux-mm@kvack.org, Andrew Morton , Peter Xu , Matthew Wilcox , "Kirill A . Shutemov" , Yang Shi , David Hildenbrand , Hugh Dickins , Jiaqi Yan , linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/khugepaged: fix iteration in collapse_file In-Reply-To: <20230607053135.2087354-1-stevensd@google.com> Message-ID: References: <20230607053135.2087354-1-stevensd@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Rspamd-Queue-Id: 7F5D3C000E X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: msy3zbr8ygqdz3s9b51ijjwnrxup6inz X-HE-Tag: 1686340990-337250 X-HE-Meta: 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 zOPt/Q8r XQrfg6tBsUGeV4d6tnzd0PCO9TmZzmz3aZhZ+BWpRUnVB9ENu4bFj1EkdXOtCov1xklN+d1nhwHCTYMaCXtl42tkHU+OUz/Rij+T4n7QgdWjMNKi0EUtSiZcjsId8RrbNeK2Kim/m4qjhTtmezkp+J3YG2sML/XjOGAjJ87uIvwTXYlzqxYT9JqxDwBBSouG4VLlxRHCJ0agM9nCTSgM2IxVsYLVruowdG8cJC9iNSNkKPT5lljgG68fiuz94M/89k9mW79ARNL0+FH05RDUo9vgcQ7kMP8pgjJgJkrzGhhMMXL3DFCCikIeIpO2+xKhEy4qnnMZKSgR/YYCXeDpAdRSxio7KtHNlkNzT2QapWCY3sQoQXzuWO8tSJHuR1YrxTu9sVZTkJeJ7Ht2yunfgz8q8jYwuBaHLZsSKGUhTg0KVA+hM+MAn3ZbWyp9f6PoEhqBbUlmofl8zFwFr7/aMzfEVJg== 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 Wed, 7 Jun 2023, David Stevens wrote: > > Remove an unnecessary call to xas_set(index) when iterating over the > target range in collapse_file. The extra call to xas_set reset the xas > cursor to the top of the tree, causing the xas_next call on the next > iteration to walk the tree to index instead of advancing to index+1. > This returned the same page again, which would cause collapse_file to > fail because the page is already locked. > > This bug was hidden when CONFIG_DEBUG_VM was set. When that config was > used, the xas_load in a subsequent VM_BUG_ON assert would walk xas from > the top of the tree to index, causing the xas_next call on the next loop > iteration to advance the cursor as expected. > > Fixes: a2e17cc2efc7 ("mm/khugepaged: maintain page cache uptodate flag") > Signed-off-by: David Stevens This patch seems to be wrong, but I have not investigated why. It's certainly an interesting and worrying observation, if a CONFIG_DEBUG_VM=y kernel goes a significantly different way. I almost always do have CONFIG_DEBUG_VM=y, so you won't be surprised that I never saw the issue. But once I ran an mm-everything with this patch in, I hit that VM_BUG_ON_PAGE(page != xas_load(&xas), page) for the first time (after about 2 hours of huge tmpfs swapping load). As if you have just transferred the problem from DEBUG_VM=n to DEBUG_VM=y. But I then tried a CONFIG_DEBUG_VM off 6.4-rc1 kernel (including the fixee but not this fixer) under similar load, and saw no problem in 14 hours. So I can't even reproduce the bug that is being fixed here: only hit a bug that it introduces. Hugh > --- > mm/khugepaged.c | 1 - > 1 file changed, 1 deletion(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 6b9d39d65b73..2d0d58fb4e7f 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -2070,7 +2070,6 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr, > TTU_IGNORE_MLOCK | TTU_BATCH_FLUSH); > > xas_lock_irq(&xas); > - xas_set(&xas, index); > > VM_BUG_ON_PAGE(page != xas_load(&xas), page); > > -- > 2.41.0.rc2.161.g9c6817b8e7-goog