Messages in this thread | | | Date | Mon, 7 Aug 2023 20:20:15 +0800 | Subject | Re: [PATCH 1/4] mm: migrate: use a folio in add_page_for_migration() | From | Kefeng Wang <> |
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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: >>>>>
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>>> >>> >>> 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
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