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[2003:cb:c73f:8500:f83c:3602:5300:88af]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4282bb98109sm249324525e9.39.2024.08.06.10.33.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 06 Aug 2024 10:33:27 -0700 (PDT) Message-ID: <907d1de9-7a38-4b6a-ba1f-eafa1b4c2783@redhat.com> Date: Tue, 6 Aug 2024 19:33:26 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/6] mm: split underutilized THPs To: Johannes Weiner Cc: Usama Arif , akpm@linux-foundation.org, linux-mm@kvack.org, riel@surriel.com, shakeel.butt@linux.dev, roman.gushchin@linux.dev, yuzhao@google.com, baohua@kernel.org, ryan.roberts@arm.com, rppt@kernel.org, willy@infradead.org, cerasuolodomenico@gmail.com, corbet@lwn.net, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, kernel-team@meta.com References: <3cd1b07d-7b02-4d37-918a-5759b23291fb@gmail.com> <73b97a03-3742-472f-9a36-26ba9009d715@gmail.com> <95ed1631-ff62-4627-8dc6-332096e673b4@redhat.com> <01899bc3-1920-4ff2-a470-decd1c282e38@gmail.com> <4b9a9546-e97b-4210-979b-262d8cf37ba0@redhat.com> <64c3746a-7b44-4dd6-a51b-e5b90557a30a@gmail.com> <204af83b-57ec-40d0-98c0-038bfeb393a3@gmail.com> <58025293-c70f-4377-b8be-39994136af83@redhat.com> <20240806172830.GD322282@cmpxchg.org> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 605D720013 X-Stat-Signature: 4bqpcdh1xyt4cbp9qukru4u1we7aqxo4 X-Rspamd-Server: rspam09 X-Rspam-User: X-HE-Tag: 1722965613-116273 X-HE-Meta: U2FsdGVkX1/4PqVQjyuFAta6xxmaDNXmVXfHBz+uRgdu1ARwu1s/5kuWwtN0W9gk8qtaFK7UgH5JpDXfxD1KT4ytsJs1exW62eb7ycVRAKOA3L6Ms0sWjt83ZPPwK+5hOlZ/TPrKFK19dj33E2V5Xi+SlXBDRMDI3XUZo5HtFSs8ou3/vSMh3L0vV9tvBvde+vBO1PLhZPZxT5LxTAoyHH/b36SWY4Afc2+mdqAlM072mVaXRbRzF1Tz7SpzCUrOOluCfwncNOb1GyuMujunc9hFFH/opdOSGtXeKJrnlJFg97Uxv4iI438tSKZde3mB6tDE5PWZwk5dn4yEH1HsZ4+alFFqu881i11VUVcwT2dsoSS4EE6EDwmD7x01i8ywORT3xC9NOne3WdYDquwwUdDeMibNsAzkEuKIVehCB6hla+c3Ajo4Wgq2ebnTgRYUf9QvhngtMNFplLf53rMzUr1YxUIPFifK0SVmafXQD/53eIhfxKe8Pv9Kyctzu75cvg2Dew3Uz2I++dQ2raOsfzw2wizr/CPj5dZVD2wfDSVpyHPgVkMDIzf7mG4rLexe7BG2UH8HGugCFUJl7bOpxwpbrEKztFIZrzW+UoefVb6ohcF5NC1rhNmFOntCQvkdMs60eoh/51tRbVzgloSyWevpnqg6f/mh7Zv9/eVeD5cblcGGpfz5ocDMyKKBrFNZc0jYFl6TnVOTyYgH5ZWa+vwEU91nRw4vloSoBpz6PHOrsp2RcyDLCRYLOVDiamuADBLebSef9bcCe6eNfNsrOVfceEtW9MU33sI+ouf6RBPw4Hky7bubdOhNNZDlE6b7/J+duzTxhV8BF4vvuJmxu2Eem66dK7AC9upZPql3otXnFSaFGKsPlTDuLLX/eLubMr9rqFu763RbgiDZV/2hN4rAVOnOOcgvHfTqDxfo/12yRXLJXntBqhRmwUWpJGBFu72WEaeanWiUhaND4mY mC6xKeC4 pA6+nnxMGv8ftRyequdrBaB3Md8go7mghKLn1SZqvf4haQxImbu8MrWr8lct8+9TZABBAocMjmqJyP62H0mbUXDGxw0VasDQCfLzox18Q2Z4HKPepXaL2KRTVhOzld7t7jLqZ91Brj9wPX+Go9GYQfi0nk2x5qIz3mOLp9osYqGRupGSewrW+P4vapdRe9D5aLEoDRfaA1KBCajZMu/D5hTf7Pb6hObinxLQbUEarw25ccAorLY81JzmgIoYQeDiZb13l/fb0t5JgJbLncTk2yNYSG2y36ToTn9NFMa7RtpICkeCIio9/VufUchka+Pbtq2OX/AL0H0qEvXF/YPgkgXom1Mr95FCWZzKUWIp9aprY0kL0qdKlFtqxeg== 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: On 06.08.24 19:28, Johannes Weiner wrote: > On Thu, Aug 01, 2024 at 08:36:32AM +0200, David Hildenbrand wrote: >> I just added another printf to postcopy_ram_supported_by_host(), where >> we temporarily do a UFFDIO_REGISTER on some test area. >> >> Sensing UFFD support # postcopy_ram_supported_by_host() >> Sensing UFFD support >> Writing received pages during precopy # ram_load_precopy() >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Disabling THP: MADV_NOHUGEPAGE # postcopy_ram_prepare_discard() >> Discarding pages # loadvm_postcopy_ram_handle_discard() >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Registering UFFD # postcopy_ram_incoming_setup() >> >> We could think about using this "ever user uffd" to avoid the shared >> zeropage in most processes. >> >> Of course, there might be other applications where that wouldn't work, >> but I think this behavior (write to area before enabling uffd) might be >> fairly QEMU specific already. > > It makes me a bit uneasy to hardcode this into the kernel. It's fairly > specific to qemu/criu, and won't protect usecases that behave slightly > differently. > > It would also give userfaultfd users that aren't susceptible to this > particular scenario a different code path. True, but let's be honest, it's not like there are a million userfaultfd users out there :) Anyhow ... > >> Avoiding the shared zeropage has the benefit that a later write fault >> won't have to do a TLB flush and can simply install a fresh anon page. > > That's true - although if that happens frequently, it's something we > might want to tune the shrinker for anyway. If subpages do get used > later, we probably shouldn't have split the THP to begin with. > > IMO the safest bet would be to use the zero page unconditionally. > ... I don't disagree. It also smells more like an optimization on top, if ever. >>> return false; >>> >>> newpte = pte_mkspecial(pfn_pte(page_to_pfn(ZERO_PAGE(pvmw->address)), >>> pvmw->vma->vm_page_prot)); >>> >>> set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte); >> >> We're replacing a present page by another present page without doing a >> TLB flush in between. I *think* this should be fine because the new >> present page is R/O and cannot possibly be written to. > > It's safe because it's replacing a migration entry. The TLB was > flushed when that was installed, and since the migration pte is not > marked present it couldn't have re-established a TLB entry. Oh, right, we're dealing with a migration entry. -- Cheers, David / dhildenb