Re: [PATCH v8 02/14] mm/gup: check every subpage of a compound page during isolation

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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[...]

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));