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Sat, 14 May 2022 00:57:02 +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 00:57:01 +0000 Message-ID: Date: Fri, 13 May 2022 17:56:59 -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 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> From: John Hubbard In-Reply-To: Content-Type: text/plain; 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dkim=pass header.d=Nvidia.com header.s=selector2 header.b=AXlHzNro; dmarc=pass (policy=reject) header.from=nvidia.com; spf=none (imf08.hostedemail.com: domain of jhubbard@nvidia.com has no SPF policy when checking 40.107.244.46) smtp.mailfrom=jhubbard@nvidia.com X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1652489809-5286 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 17:26, 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 */ return NULL; ...and now I'm starting to think that this warning might fire even with the corrected check for MIGRATE_CMA || MIGRATE_ISOLATE. Because try_grab_folio() didn't always have this early exit and it is starting to look wrong. Simply attempting to pin a non-pinnable huge page would hit this warning. Adding additional reasons that a page is not pinnable (which the patch does) could make this more likely to fire. I need to look at this a little more closely, it is making me wonder whether the is_pinnable_page() check is a problem in this path. The comment in try_grab_folio() indicates that the early return is a hack (it assumes that the caller is in the gup fast path), and maybe the hack is just wrong here--I think we're actually on the slow gup path. Not good. Mike, any thoughts here? thanks, -- John Hubbard NVIDIA