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V" , Mike Kravetz Subject: Re: [PATCH v3 12/12] mm/gup: Handle hugetlb in the generic follow_page_mask code Message-ID: References: <20240321220802.679544-1-peterx@redhat.com> <20240321220802.679544-13-peterx@redhat.com> <20240322133012.GI159172@nvidia.com> MIME-Version: 1.0 In-Reply-To: <20240322133012.GI159172@nvidia.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Rspamd-Queue-Id: 4491D1A0016 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: xki5mpgakbkwqfnym57j4mjgyqyjeykg X-HE-Tag: 1711122919-454561 X-HE-Meta: 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 +5kn/2A4 wdToawSgH7PH5pL9QiUuDk+zCZImE8gq39sSQl8rG3nmYNkN9M4QPGmEg0Bix5hskmm7oEta1/8wn04JLFLyzi/F8iIc9sC6t2/0wXCFJa0lUoRvHuIzlE+ZarDOyKhD4Tj94bRDgG7thhSoR/QhK55Jyw3QdQGx7o3F7bSRfEvd3aIuu/AekTzRoOgac2BzUtWVWbBvn+fbgW/ES6vaF2lrKmkwpYALZmjNp2FjDK0JHG5Ujp2flKYK/fMRaV6bbLmcSFt31VbUptDzfUPwD5QopVPNieUku95+PywdUNzhIxVfNAfSZ9YlJILfIt/YX5qek2R61O7sN2/SsdpxsqpWLqhbKP+uEEMIsSnrB3a75TyAHIgarzhqSLNcqCtcRmf6VJhIDLUOJajR2ISRbBDuWq/wYt+Ve/FbWCRuGIsqXqxpxFk09zok6cFEMWtSiiPNpntykbMo48hY= 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: List-Subscribe: List-Unsubscribe: Jason, On Fri, Mar 22, 2024 at 10:30:12AM -0300, Jason Gunthorpe wrote: > On Thu, Mar 21, 2024 at 06:08:02PM -0400, peterx@redhat.com wrote: > > > A quick performance test on an aarch64 VM on M1 chip shows 15% degrade over > > a tight loop of slow gup after the path switched. That shouldn't be a > > problem because slow-gup should not be a hot path for GUP in general: when > > page is commonly present, fast-gup will already succeed, while when the > > page is indeed missing and require a follow up page fault, the slow gup > > degrade will probably buried in the fault paths anyway. It also explains > > why slow gup for THP used to be very slow before 57edfcfd3419 ("mm/gup: > > accelerate thp gup even for "pages != NULL"") lands, the latter not part of > > a performance analysis but a side benefit. If the performance will be a > > concern, we can consider handle CONT_PTE in follow_page(). > > I think this is probably fine for the moment, at least for this > series, as CONT_PTE is still very new. > > But it will need to be optimized. "slow" GUP is the only GUP that is > used by FOLL_LONGTERM and it still needs to be optimized because you > can't assume a FOLL_LONGTERM user will be hitting the really slow > fault path. There are enough important cases where it is just reading > already populted page tables, and these days, often with large folios. Ah, I thought FOLL_LONGTERM should work in most cases for fast-gup, especially for hugetlb, but maybe I missed something? I do see that devmap skips fast-gup for LONGTERM, we also have that writeback issue but none of those that I can find applies to hugetlb. This might be a problem indeed if we have hugetlb cont_pte pages that will constantly fallback to slow gup. OTOH, I also agree with you that such batching would be nice to have for slow-gup, likely devmap or many fs (exclude shmem/hugetlb) file mappings can at least benefit from it due to above. But then that'll be a more generic issue to solve, IOW, we still don't do that for !hugetlb cont_pte large folios, before or after this series. > > Reviewed-by: Jason Gunthorpe Thanks! -- Peter Xu