• Andrea Arcangeli's avatar
    mm: thp: fix /dev/zero MAP_PRIVATE and vm_flags cleanups · 78f11a25
    Andrea Arcangeli authored
    The huge_memory.c THP page fault was allowed to run if vm_ops was null
    (which would succeed for /dev/zero MAP_PRIVATE, as the f_op->mmap wouldn't
    setup a special vma->vm_ops and it would fallback to regular anonymous
    memory) but other THP logics weren't fully activated for vmas with vm_file
    not NULL (/dev/zero has a not NULL vma->vm_file).
    
    So this removes the vm_file checks so that /dev/zero also can safely use
    THP (the other albeit safer approach to fix this bug would have been to
    prevent the THP initial page fault to run if vm_file was set).
    
    After removing the vm_file checks, this also makes huge_memory.c stricter
    in khugepaged for the DEBUG_VM=y case.  It doesn't replace the vm_file
    check with a is_pfn_mapping check (but it keeps checking for VM_PFNMAP
    under VM_BUG_ON) because for a is_cow_mapping() mapping VM_PFNMAP should
    only be allowed to exist before the first page fault, and in turn when
    vma->anon_vma is null (so preventing khugepaged registration).  So I tend
    to think the previous comment saying if vm_file was set, VM_PFNMAP might
    have been set and we could still be registered in khugepaged (despite
    anon_vma was not NULL to be registered in khugepaged) was too paranoid.
    The is_linear_pfn_mapping check is also I think superfluous (as described
    by comment) but under DEBUG_VM it is safe to stay.
    
    Addresses https://bugzilla.kernel.org/show_bug.cgi?id=33682Signed-off-by: default avatarAndrea Arcangeli <aarcange@redhat.com>
    Reported-by: default avatarCaspar Zhang <bugs@casparzhang.com>
    Acked-by: default avatarMel Gorman <mel@csn.ul.ie>
    Acked-by: default avatarRik van Riel <riel@redhat.com>
    Cc: <stable@kernel.org>		[2.6.38.x]
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    78f11a25
huge_memory.c 61.7 KB