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[85.23.48.6]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-55638497801sm1163807e87.127.2025.07.06.20.29.36 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 06 Jul 2025 20:29:37 -0700 (PDT) Message-ID: Date: Mon, 7 Jul 2025 06:29:36 +0300 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [v1 resend 08/12] mm/thp: add split during migration support To: Balbir Singh , linux-mm@kvack.org Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org, Karol Herbst , Lyude Paul , Danilo Krummrich , David Airlie , Simona Vetter , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Shuah Khan , David Hildenbrand , Barry Song , Baolin Wang , Ryan Roberts , Matthew Wilcox , Peter Xu , Zi Yan , Kefeng Wang , Jane Chu , Alistair Popple , Donet Tom References: <20250703233511.2028395-1-balbirs@nvidia.com> <20250703233511.2028395-9-balbirs@nvidia.com> <715fc271-1af3-4061-b217-e3d6e32849c6@redhat.com> <6bb5b0ae-6de3-4df8-a8c1-07d4c6f8c275@redhat.com> <2ace3c9e-452a-4bd2-a7df-6fa9fd3de290@nvidia.com> From: =?UTF-8?Q?Mika_Penttil=C3=A4?= In-Reply-To: <2ace3c9e-452a-4bd2-a7df-6fa9fd3de290@nvidia.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: KOS1VfSsNAn2LJuPVMaXI3jm4vScu0m2CH_V3zn0Ims_1751858979 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 457434000F X-Stat-Signature: ujd6yebhpzxkenf1dcun7pkoyk8w9px4 X-Rspam-User: X-HE-Tag: 1751858983-210972 X-HE-Meta: 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 s6b522++ xH1UiOQTndAALh8ZpAwa0aWSq/ziWq8FWYDFuBE6AD6az5q+ELTQ7qLxR0Jv6Hm72ZXeBTZAMqTG7SwMTc+cahcNY8MkybA20Fo5gg/Y88JbjqQ4/LoXgiA0kwtiFCQ7QxCMOJXZjJuOh+96n5oX5pQyHJ5H1RXMCapNSJvJGf5C5BMVhAj/ZnCv4H7dWQaz2SiplHjHYfIP3Q1MOSk8odLE5q7OCc98zW5NMAOY5/fXitJ1ylZdJjw0XnbXwsadX4yjuyIl2MoZYXS6ufmgz2B+DWcNkPZLtYj8FCR69iybQ02TV57Gc2h5RlcbenwRr5CE8RG3Ww48qjQToDvqmE48A603rE6UdSdnjBU9aYGplziKfRxZd0nX8QcRPZME6ADzDibLBPJP8Fvpj6ZsdoG8pxw== 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 7/7/25 05:35, Balbir Singh wrote: > On 7/5/25 13:17, Mika Penttilä wrote: >>>>>> +static void migrate_vma_split_pages(struct migrate_vma *migrate, >>>>>> + unsigned long idx, unsigned long addr, >>>>>> + struct folio *folio) >>>>>> +{ >>>>>> + unsigned long i; >>>>>> + unsigned long pfn; >>>>>> + unsigned long flags; >>>>>> + >>>>>> + folio_get(folio); >>>>>> + split_huge_pmd_address(migrate->vma, addr, true); >>>>>> + __split_huge_page_to_list_to_order(folio_page(folio, 0), NULL, 0, true); >>>>> We already have reference to folio, why is folio_get() needed ? >>>>> >>>>> Splitting the page splits pmd for anon folios, why is there split_huge_pmd_address() ? >>>> Oh I see >>>> + if (!isolated) >>>> + unmap_folio(folio); >>>> >>>> which explains the explicit split_huge_pmd_address(migrate->vma, addr, true); >>>> >>>> Still, why the folio_get(folio);? >>>> >>>> >>> That is for split_huge_pmd_address, when called with freeze=true, it drops the >>> ref count on the page >>> >>> if (freeze) >>> put_page(page); >>> >>> Balbir >>> >> yeah I guess you could have used the pmd_migration path in __split_huge_pmd_locked, and not use freeze because you have installed the migration pmd entry already. >> Which brings to a bigger concern, that you do need the freeze semantics, like clear PageAnonExclusive (which may fail). I think you did not get this part >> right in the 3/12 patch. And in this patch, you can't assume the split succeeds, which would mean you can't migrate the range at all. >> Doing the split this late is quite problematic all in all. >> > Clearing PageAnonExclusive will *not* fail for device private pages from what I can see in __folio_try_share_anon_rmap(). > Doing the split late is a requirement due to the nature of the three stage migration operation, the other side > might fail to allocate THP sized pages and so the code needs to deal with it > > Balbir Singh Yes seems clearing PageAnonExclusive doesn't fail for device private pages in the end, but the 3/12 patch doesn't even try to clear PageAnonExclusive with your changes afaics, which is a separate issue. And __split_huge_page_to_list_to_order() (return value is not checked) can fail for out of memory. So think you can not just assume split just works. If late split is a requirement (which I can understand is), you should be prepared to rollback somehow the operation. > --Mika