8d63d99a5d
THP uses tail page refcounting to be able to split huge pages at any time. Tail page refcounting is not needed for other users of compound pages and it's harmful because of overhead. We try to exclude non-THP pages from tail page refcounting using __compound_tail_refcounted() check. It excludes most common non-THP compound pages: SL*B and hugetlb, but it doesn't catch rest of __GFP_COMP users -- drivers. And it's not only about overhead. Drivers might want to use compound pages to get refcounting semantics suitable for mapping high-order pages to userspace. But tail page refcounting breaks it. Tail page refcounting uses ->_mapcount in tail pages to store GUP pins on them. It means GUP pins would affect page_mapcount() for tail pages. It's not a problem for THP, because it never maps tail pages. But unlike THP, drivers map parts of compound pages with PTEs and it makes page_mapcount() be called for tail pages. In particular, GUP pins would shift PSS up and affect /proc/kpagecount for such pages. But, I'm not aware about anything which can lead to crash or other serious misbehaviour. Since currently all THP pages are anonymous and all drivers pages are not, we can fix the __compound_tail_refcounted() check by requiring PageAnon() to enable tail page refcounting. Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Acked-by: Hugh Dickins <hughd@google.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
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