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    mm: page_mkclean vs MADV_DONTNEED race · 38c5fce7
    Aneesh Kumar K.V authored
    [ Upstream commit 024eee0e ]
    
    MADV_DONTNEED is handled with mmap_sem taken in read mode.  We call
    page_mkclean without holding mmap_sem.
    
    MADV_DONTNEED implies that pages in the region are unmapped and subsequent
    access to the pages in that range is handled as a new page fault.  This
    implies that if we don't have parallel access to the region when
    MADV_DONTNEED is run we expect those range to be unallocated.
    
    w.r.t page_mkclean() we need to make sure that we don't break the
    MADV_DONTNEED semantics.  MADV_DONTNEED check for pmd_none without holding
    pmd_lock.  This implies we skip the pmd if we temporarily mark pmd none.
    Avoid doing that while marking the page clean.
    
    Keep the sequence same for dax too even though we don't support
    MADV_DONTNEED for dax mapping
    
    The bug was noticed by code review and I didn't observe any failures w.r.t
    test run.  This is similar to
    
    commit 58ceeb6b
    Author: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Date:   Thu Apr 13 14:56:26 2017 -0700
    
        thp: fix MADV_DONTNEED vs. MADV_FREE race
    
    commit ced10803
    Author: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Date:   Thu Apr 13 14:56:20 2017 -0700
    
        thp: fix MADV_DONTNEED vs. numa balancing race
    
    Link: http://lkml.kernel.org/r/20190321040610.14226-1-aneesh.kumar@linux.ibm.com
    
    
    Signed-off-by: default avatarAneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
    Reviewed-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Cc: Dan Williams <dan.j.williams@intel.com>
    Cc:"Kirill A . Shutemov" <kirill@shutemov.name>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: default avatarSasha Levin <sashal@kernel.org>
    38c5fce7