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=-0.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=no 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 B72EDC4BA24 for ; Thu, 27 Feb 2020 01:56:17 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 5596924672 for ; Thu, 27 Feb 2020 01:56:17 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="zvExgfUB" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5596924672 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=linux-foundation.org Authentication-Results: mail.kernel.org; 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Thu, 27 Feb 2020 01:56:15 +0000 (UTC) Received: from localhost.localdomain (c-73-231-172-41.hsd1.ca.comcast.net [73.231.172.41]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id CA56224670; Thu, 27 Feb 2020 01:56:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582768575; bh=nrMQrZPfEdRrjSle80bMH/KoGs//luBWZZY7OVIw48g=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=zvExgfUBOtwOgyugclggGNkUn3WQvJBx+q//q3lce8vxGBTDFCw1m0LeUK6JlgNm6 7N8ObfeEZwQ8OAnqH2VJNqBXODf36XsKuzXy3h6wA/E7APIf+nBuSnu++0BRGifx9y gRpUFu6bpQbLqzmoLuj2qK5w8xyWGbs/KhLQK63k= Date: Wed, 26 Feb 2020 17:56:14 -0800 From: Andrew Morton To: Vlastimil Babka Cc: Dan Carpenter , mgorman@techsingularity.net, linux-mm@kvack.org Subject: Re: [bug report] mm, compaction: capture a page under direct compaction Message-Id: <20200226175614.5dfe21b53cad9f8bca084a2b@linux-foundation.org> In-Reply-To: <82147552-be4c-84cc-53d8-f00711859427@suse.cz> References: <20200224062949.mhyi2ntdiq4vbptm@kili.mountain> <82147552-be4c-84cc-53d8-f00711859427@suse.cz> X-Mailer: Sylpheed 3.5.1 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit 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, 26 Feb 2020 18:04:56 +0100 Vlastimil Babka wrote: > On 2/24/20 7:35 AM, Dan Carpenter wrote: > > Hello Mel Gorman, > > > > This is a semi-automatic email about new static checker warnings. > > > > The patch 5e1f0f098b46: "mm, compaction: capture a page under direct > > compaction" from Mar 5, 2019, leads to the following Smatch complaint: > > > > mm/compaction.c:2321 compact_zone_order() > > error: we previously assumed 'capture' could be null (see line 2313) > > > > ----8<---- > >From fb264bb52576a0582e8a92952cf91e8b61d105c3 Mon Sep 17 00:00:00 2001 > From: Vlastimil Babka > Date: Wed, 26 Feb 2020 17:53:54 +0100 > Subject: [PATCH] mm, compaction: fully assume capture can be NULL in > compact_zone_order() > > Dan reports: > > The patch 5e1f0f098b46: "mm, compaction: capture a page under direct > compaction" from Mar 5, 2019, leads to the following Smatch complaint: > > mm/compaction.c:2321 compact_zone_order() > error: we previously assumed 'capture' could be null (see line 2313) > > mm/compaction.c > 2288 static enum compact_result compact_zone_order(struct zone *zone, int order, > 2289 gfp_t gfp_mask, enum compact_priority prio, > 2290 unsigned int alloc_flags, int classzone_idx, > 2291 struct page **capture) > ^^^^^^^ > > 2313 if (capture) > ^^^^^^^ > Check for NULL > > 2314 current->capture_control = &capc; > 2315 > 2316 ret = compact_zone(&cc, &capc); > 2317 > 2318 VM_BUG_ON(!list_empty(&cc.freepages)); > 2319 VM_BUG_ON(!list_empty(&cc.migratepages)); > 2320 > 2321 *capture = capc.page; > ^^^^^^^^ > Unchecked dereference. > > 2322 current->capture_control = NULL; > 2323 > > In practice this is not an issue, as the only caller path passes non-NULL > capture: > > __alloc_pages_direct_compact() > struct page *page = NULL; > try_to_compact_pages(capture = &page); > compact_zone_order(capture = capture); > > Hence no stable tag needed, but let's make this more robust and remove the > warning by assuming capture might be NULL in the second dereference. > > ... > > --- mm/compaction.c > +++ mm/compaction.c > @@ -2318,8 +2318,10 @@ static enum compact_result compact_zone_order(struct zone *zone, int order, > VM_BUG_ON(!list_empty(&cc.freepages)); > VM_BUG_ON(!list_empty(&cc.migratepages)); > > - *capture = capc.page; > - current->capture_control = NULL; > + if (capture) { > + *capture = capc.page; > + current->capture_control = NULL; > + } > > return ret; > } Why don't we instead remove the first check for NULL? Remove some dead code instead of adding some?