• Andrea Arcangeli's avatar
    mm: thp: fix pmd_present for split_huge_page and PROT_NONE with THP · 027ef6c8
    Andrea Arcangeli authored
    In many places !pmd_present has been converted to pmd_none.  For pmds
    that's equivalent and pmd_none is quicker so using pmd_none is better.
    
    However (unless we delete pmd_present) we should provide an accurate
    pmd_present too.  This will avoid the risk of code thinking the pmd is non
    present because it's under __split_huge_page_map, see the pmd_mknotpresent
    there and the comment above it.
    
    If the page has been mprotected as PROT_NONE, it would also lead to a
    pmd_present false negative in the same way as the race with
    split_huge_page.
    
    Because the PSE bit stays on at all times (both during split_huge_page and
    when the _PAGE_PROTNONE bit get set), we could only check for the PSE bit,
    but checking the PROTNONE bit too is still good to remember pmd_present
    must always keep PROT_NONE into account.
    
    This explains a not reproducible BUG_ON that was seldom reported on the
    lists.
    
    The same issue is in pmd_large, it would go wrong with both PROT_NONE and
    if it races with split_huge_page.
    Signed-off-by: default avatarAndrea Arcangeli <aarcange@redhat.com>
    Acked-by: default avatarRik van Riel <riel@redhat.com>
    Cc: Johannes Weiner <jweiner@redhat.com>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Mel Gorman <mgorman@suse.de>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    027ef6c8
pgtable.h 18.6 KB