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charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.177.243] X-ClientProxiedBy: dggems703-chm.china.huawei.com (10.3.19.180) To dggpemm100001.china.huawei.com (7.185.36.93) X-CFilter-Loop: Reflected X-Rspamd-Queue-Id: B609B40004 X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: 63p4aqqkp5bu54b9odwo85jn1jmdh8ao X-HE-Tag: 1691410823-336546 X-HE-Meta: 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 AJAMLUQM tF/8uqOuzFGFN8c0FDiIi8GdVH6OSNgmsHQWOoVGkK8maHg32vpgidJOnwHpRoyFHirtglrycC6KOY4Nf8d/JYJFID/blwyY0azT2CBW65LmeTEUUgkbOH5AXJuZy/sKUOcKAq6/mgcQr7UMsl5b1yoh8n7uk/MQL6QASyw1PLIZW3Yg70mk33I4PQkwb3jCA63TG0ONb2X5MQURqstxK1GZGiejx5xHVGjW/ka4VRgQOWlqoa8PwW95lfHuBSgoDGSteqcYPP2ZKM12WhBPFV2HheaVvm4tsi0GIM5pIgGokbyB76OecTNPIAw== 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: Hi Zi Yan and Matthew and Naoya, On 2023/8/4 13:54, Kefeng Wang wrote: > > > On 2023/8/4 10:42, Zi Yan wrote: >> On 3 Aug 2023, at 21:45, Kefeng Wang wrote: >> >>> On 2023/8/3 20:30, Matthew Wilcox wrote: >>>> On Thu, Aug 03, 2023 at 03:13:21PM +0800, Kefeng Wang wrote: >>>>> ... >>> >>> >>>    if (PageHuge(page))  // page must be a hugetlb page >>>     if (PageHead(page)) // page must be a head page, not tail >>>               isolate_hugetlb() // isolate the hugetlb page if head >>> >>> After using folio, >>> >>>    if (folio_test_hugetlb(folio)) // only check folio is hugetlb or not >>> >>> I don't check the page is head or not, since the follow_page could >>> return a sub-page, so the check PageHead need be retained, right? >> >> Right. It will prevent the kernel from trying to isolate the same >> hugetlb page >> twice when two pages are in the same hugetlb folio. But looking at the >> code, if you try to isolate an already-isolated hugetlb folio, >> isolate_hugetlb() >> would return false, no error would show up. But it changes err value >> from -EACCES to -EBUSY and user will see a different page status than >> before. > Before e66f17ff7177 ("mm/hugetlb: take page table lock in follow_huge_pmd()") in v4.0, follow_page() will return NULL on tail page for Huagetlb page, and move_pages() will return -ENOENT errno,but after that commit, -EACCES is returned, which not match the manual, > > When check man[1], the current -EACCES is not right, -EBUSY is not > precise but more suitable for this scenario, > >      -EACCES >               The page is mapped by multiple processes and can be moved >               only if MPOL_MF_MOVE_ALL is specified. > >      -EBUSY The page is currently busy and cannot be moved.  Try again >               later.  This occurs if a page is undergoing I/O or another >               kernel subsystem is holding a reference to the page. >     -ENOENT >               The page is not present. > >> >> I wonder why we do not have follow_folio() and returns -ENOENT error >> pointer >> when addr points to a non head page. It would make this patch more >> folio if >> follow_folio() can be used in place of follow_page(). One caveat is that >> user will see -ENOENT instead of -EACCES after this change. >> > > -ENOENT is ok, but maybe the man need to be updated too. According to above analysis, -ENOENT is suitable when introduce the follow_folio(), but when THP migrate support is introduced by e8db67eb0ded ("mm: migrate: move_pages() supports thp migration") in v4.14, the tail page will be turned into head page and return -EBUSY, So should we unify errno(maybe use -ENOENT) about the tail page? > > > > [1] https://man7.org/linux/man-pages/man2/move_pages.2.html