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[2003:cb:c707:c500:5445:cf40:2e32:6e73]) by smtp.gmail.com with ESMTPSA id bt2-20020a056000080200b002255eebf785sm6594605wrb.89.2022.08.24.04.55.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 24 Aug 2022 04:55:50 -0700 (PDT) Message-ID: <041e2e43-2227-1681-743e-5f82e245b5ea@redhat.com> Date: Wed, 24 Aug 2022 13:55:49 +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 Content-Language: en-US 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> From: David Hildenbrand Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? -- Thanks, David / dhildenb