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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 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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