Messages in this thread | | | Subject | Re: [PATCH v8 02/14] mm/gup: check every subpage of a compound page during isolation | From | Joao Martins <> | Date | Wed, 3 Feb 2021 13:22:18 +0000 |
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On 1/25/21 7:47 PM, Pavel Tatashin wrote: > When pages are isolated in check_and_migrate_movable_pages() we skip > compound number of pages at a time. However, as Jason noted, it is > not necessary correct that pages[i] corresponds to the pages that > we skipped. This is because it is possible that the addresses in > this range had split_huge_pmd()/split_huge_pud(), and these functions > do not update the compound page metadata. > > The problem can be reproduced if something like this occurs: > > 1. User faulted huge pages. > 2. split_huge_pmd() was called for some reason > 3. User has unmapped some sub-pages in the range > 4. User tries to longterm pin the addresses. > > The resulting pages[i] might end-up having pages which are not compound > size page aligned. > > Fixes: aa712399c1e8 ("mm/gup: speed up check_and_migrate_cma_pages() on huge page") > Reported-by: Jason Gunthorpe <jgg@nvidia.com> > Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com> > Reviewed-by: Jason Gunthorpe <jgg@nvidia.com> > ---
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> /* > * If we get a page from the CMA zone, since we are going to > * be pinning these entries, we might as well move them out > @@ -1599,8 +1596,6 @@ static long check_and_migrate_cma_pages(struct mm_struct *mm, > } > } > } > - > - i += step; > } >
With this, longterm gup will 'regress' for hugetlbfs e.g. from ~6k -> 32k usecs when pinning a 16G hugetlb file.
Splitting can only occur on THP right? If so, perhaps we could retain the @step increment for compound pages but when !is_transparent_hugepage(head) or just PageHuge(head) like:
+ if (!is_transparent_hugepage(head) && PageCompound(page)) + i += (compound_nr(head) - (pages[i] - head));
Or making specific to hugetlbfs:
+ if (PageHuge(head)) + i += (compound_nr(head) - (pages[i] - head));
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