Messages in this thread | | | Date | Wed, 9 Aug 2023 20:37:15 +0800 | Subject | Re: [PATCH 1/4] mm: migrate: use a folio in add_page_for_migration() | From | Kefeng Wang <> |
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Hi Mike
On 2023/8/8 2:45, Zi Yan wrote: > On 7 Aug 2023, at 8:20, Kefeng Wang wrote: > >> 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 > > I think so. I think -EBUSY is more reasonable for tail pages. But there is > some subtle difference between THP and hugetlb from current code: > > For THP, compound_head() is used to get the head page for isolation, this means > if user specifies a tail page address in move_pages(), the whole THP can be > migrated. > > For hugetlb, only if user specifies the head page address of a hugetlb page, > the hugetlb page will be migrated. Otherwise, an error would show up. > > Cc Mike to help us clarify the expected behavior of hugetlb. > > Hi Mike, what is the expected behavior, if a user tries to use move_pages() > to migrate a non head page of a hugetlb page?
Could you give some advise, thanks
> > -- > Best Regards, > Yan, Zi
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