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[2003:cb:c707:c500:5445:cf40:2e32:6e73]) by smtp.gmail.com with ESMTPSA id j19-20020a05600c1c1300b003a5537bb2besm2295491wms.25.2022.08.24.07.33.25 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 24 Aug 2022 07:33:26 -0700 (PDT) Message-ID: <0f736dc5-1798-10ad-c506-9a2a38841359@redhat.com> Date: Wed, 24 Aug 2022 16:33:25 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.12.0 Subject: Re: [PATCH v2 1/5] mm/hugetlb: fix races when looking up a CONT-PTE size hugetlb page To: Baolin Wang , Mike Kravetz Cc: akpm@linux-foundation.org, songmuchun@bytedance.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <0e5d92da043d147a867f634b17acbcc97a7f0e64.1661240170.git.baolin.wang@linux.alibaba.com> <4c24b891-04ce-2608-79d2-a75dc236533f@redhat.com> <376d2e0a-d28a-984b-903c-1f6451b04a15@linux.alibaba.com> <7d4e7f47-30a5-3cc6-dc9f-aa89120847d8@redhat.com> <64669c0a-4a6e-f034-a15b-c4a8deea9e5d@linux.alibaba.com> <7ee73879-e402-9175-eae8-41471d80d59e@redhat.com> <041e2e43-2227-1681-743e-5f82e245b5ea@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1661351610; a=rsa-sha256; cv=none; b=jLJJE1I1X8Owc5j8ErmRKJMEKT3W1w9W+ggMIYX3w+SuAloIByAH2OJ0nQ1SjA5vci0wU1 wHhvsrBvfYIxRKSq6Qjn4FdZUclp8VmjLMQaYzJkkpoWvQ1bop5mM945la+cWoaXqb/brs dzbP0IkGGS4A4HFWTt8VsK82Q6QsIwk= ARC-Authentication-Results: i=1; imf13.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=PNy3hTsD; spf=pass (imf13.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1661351610; 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:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=YmgeeKGw4Tl+BHIoGUkJ+Zr3UgoFX8dfwLgZq2Sqd+E=; b=IKOLuQXNRycEX0sDk+MHhEVXUMb+USrwlKZgrQ/dIkWCXxKkG0wJMs95BX37dkosxjmxmx XewgOUOCgr1MBpqVR2JxBubjYPsd755tnhKnA84qypohiLUVXSH22rsKODIbPlLCKpPKrK UU/B7opnYsClpj/LcTfa6dGk335smyA= X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 00CA120016 Authentication-Results: imf13.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=PNy3hTsD; spf=pass (imf13.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Stat-Signature: fk4mayogp3gztcbxst3byybp6t978agi X-HE-Tag: 1661351609-705083 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 24.08.22 16:30, Baolin Wang wrote: > > > On 8/24/2022 7:55 PM, David Hildenbrand wrote: >> On 24.08.22 11:41, Baolin Wang wrote: >>> >>> >>> On 8/24/2022 3:31 PM, David Hildenbrand wrote: >>>>>>>> >>>>>>>> IMHO, these follow_huge_xxx() functions are arch-specified at first and >>>>>>>> were moved into the common hugetlb.c by commit 9e5fc74c3025 ("mm: >>>>>>>> hugetlb: Copy general hugetlb code from x86 to mm"), and now there are >>>>>>>> still some arch-specified follow_huge_xxx() definition, for example: >>>>>>>> ia64: follow_huge_addr >>>>>>>> powerpc: follow_huge_pd >>>>>>>> s390: follow_huge_pud >>>>>>>> >>>>>>>> What I mean is that follow_hugetlb_page() is a common and >>>>>>>> not-arch-specified function, is it suitable to change it to be >>>>>>>> arch-specified? >>>>>>>> And thinking more, can we rename follow_hugetlb_page() as >>>>>>>> hugetlb_page_faultin() and simplify it to only handle the page faults of >>>>>>>> hugetlb like the faultin_page() for normal page? That means we can make >>>>>>>> sure only follow_page_mask() can handle hugetlb. >>>>>>>> >>>>>> >>>>>> Something like that might work, but you still have two page table walkers >>>>>> for hugetlb. I like David's idea (if I understand it correctly) of >>>>> >>>>> What I mean is we may change the hugetlb handling like normal page: >>>>> 1) use follow_page_mask() to look up a hugetlb firstly. >>>>> 2) if can not get the hugetlb, then try to page fault by >>>>> hugetlb_page_faultin(). >>>>> 3) if page fault successed, then retry to find hugetlb by >>>>> follow_page_mask(). >>>> >>>> That implies putting more hugetlbfs special code into generic GUP, >>>> turning it even more complicated. But of course, it depends on how the >>>> end result looks like. My gut feeling was that hugetlb is better handled >>>> in follow_hugetlb_page() separately (just like we do with a lot of other >>>> page table walkers). >>> >>> OK, fair enough. >>> >>>>> >>>>> Just a rough thought, and I need more investigation for my idea and >>>>> David's idea. >>>>> >>>>>> using follow_hugetlb_page for both cases. As noted, it will need to be >>>>>> taught how to not trigger faults in the follow_page_mask case. >>>>> >>>>> Anyway, I also agree we need some cleanup, and firstly I think we should >>>>> cleanup these arch-specified follow_huge_xxx() on some architectures >>>>> which are similar with the common ones. I will look into these. >>>> >>>> There was a recent discussion on that, e.g.: >>>> >>>> https://lkml.kernel.org/r/20220818135717.609eef8a@thinkpad >>> >>> Thanks. >>> >>>> >>>>> >>>>> However, considering cleanup may need more investigation and >>>>> refactoring, now I prefer to make these bug-fix patches of this patchset >>>>> into mainline firstly, which are suitable to backport to old version to >>>>> fix potential race issues. Mike and David, how do you think? Could you >>>>> help to review these patches? Thanks. >>>> >>>> Patch #1 certainly add more special code just to handle another hugetlb >>>> corner case (CONT pages), and maybe just making it all use >>>> follow_hugetlb_page() would be even cleaner and less error prone. >>>> >>>> I agree that locking is shaky, but I'm not sure if we really want to >>>> backport this to stable trees: >>>> >>>> https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html >>>> >>>> "It must fix a real bug that bothers people (not a, “This could be a >>>> problem...” type thing)." >>>> >>>> >>>> Do we actually have any instance of this being a real (and not a >>>> theoretical) problem? If not, I'd rather clean it all up right away. >>> >>> I think this is a real problem (not theoretical), and easy to write some >>> code to show the issue. For example, suppose thread A is trying to look >>> up a CONT-PTE size hugetlb page under the lock, however antoher thread B >>> can migrate the CONT-PTE hugetlb page at the same time, which will cause >>> thread A to get an incorrect page, if thread A want to do something for >>> this incorrect page, error occurs. >>> >>> Actually we also want to backport these fixes to the distro with old >>> kernel versions to make the hugetlb more stable. Otherwise we must hit >>> these issues sooner or later if the customers use CONT-PTE/PMD hugetlb. >>> >>> Anyway, if you and Mike still think these issues are not important >>> enough to be fixed in the old versions, I can do the cleanup firstly. >>> >> >> [asking myself which follow_page() users actually care about hugetlb, >> and why we need this handling in follow_page at all] >> >> Which follow_page() user do we care about here? Primarily mm/migrate.c >> only I assume? > > Right, mainly affects the move_pages() syscall I think. Yes, I can not > know all of the users of the move_pages() syscall now or in the future > in our data center, but like I said the move_pages() syscall + hugetlb > can be a real potential stability issue. > I wonder if we can get rid of follow_page() completely, there are not too many users. Or alternatively simply make it use general GUP infrastructure more clearly. We'd need something like FOLL_NOFAULT that also covers "absolutely no faults". -- Thanks, David / dhildenb