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Sat, 14 May 2022 01:16:48 +0000 Received: from BY5PR12MB4130.namprd12.prod.outlook.com ([fe80::15e3:d746:57e8:4de0]) by BY5PR12MB4130.namprd12.prod.outlook.com ([fe80::15e3:d746:57e8:4de0%7]) with mapi id 15.20.5250.017; Sat, 14 May 2022 01:16:48 +0000 Message-ID: <1c6405b0-271e-22b8-961e-629a2a8bbcf0@nvidia.com> Date: Fri, 13 May 2022 18:16:46 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.9.0 Subject: Re: [syzbot] WARNING in follow_hugetlb_page Content-Language: en-US From: John Hubbard To: Minchan Kim Cc: Andrew Morton , Mike Kravetz , syzbot , linux-kernel@vger.kernel.org, linux-mm@kvack.org, llvm@lists.linux.dev, nathan@kernel.org, ndesaulniers@google.com, syzkaller-bugs@googlegroups.com, trix@redhat.com, Matthew Wilcox , Stephen Rothwell , David Hildenbrand References: <000000000000ef451a05dee0f2b1@google.com> <00000000000077377c05dee75f63@google.com> <20220513102617.c464c4f566052838e911a3ec@linux-foundation.org> <75f09063-d184-7d44-17a1-ed04be5eb953@oracle.com> <20220513161910.d1b73583cdb2e33562aa86e5@linux-foundation.org> <4809b134-a37a-50b8-4c25-44548bc1048f@nvidia.com> In-Reply-To: Content-Type: text/plain; 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dkim=pass header.d=Nvidia.com header.s=selector2 header.b="s988/GBv"; spf=none (imf10.hostedemail.com: domain of jhubbard@nvidia.com has no SPF policy when checking 40.107.101.49) smtp.mailfrom=jhubbard@nvidia.com; dmarc=pass (policy=reject) header.from=nvidia.com X-HE-Tag: 1652490985-40446 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 5/13/22 17:56, John Hubbard wrote: > On 5/13/22 17:26, Minchan Kim wrote: >> Anything else further can we get insight from the warning? >> >> For example, pin_user_pages going on against a hugetlb page >> which are concurrently running alloc_contig_range(it's >> exported function so anyone can call randomly) so >> alloc_contig_range changes pageblock type as MIGRATE_ISOLATE >> under us so the hit at the warning? > > Well, yes. First of all, the comments above the warning that fired have > gone a little bit stale: they claim that we can only hit the warning if > the page refcount overflows. However, we almost certainly got here via: > > try_grab_folio() >     /* >      * Can't do FOLL_LONGTERM + FOLL_PIN gup fast path if not in a >      * right zone, so fail and let the caller fall back to the slow >      * path. >      */ >     if (unlikely((flags & FOLL_LONGTERM) && >              !is_pinnable_page(page))) /* which we just changed */ Specifically, the recent patch effectively acted as an error injection test, by forcing is_pinnable_page() to always return true (if CONFIG_CMA is defined). Because: MIGRATE_CMA|MIGRATE_ISOLATE == 7, which will match any of the MIGRATE_* enums when checked with bitwise AND. I suspect this particular error path has not been exercised much, or if it has, not reported here anyway. Until now. thanks, -- John Hubbard NVIDIA