• Yang Shi's avatar
    mm: mremap: downgrade mmap_sem to read when shrinking · 85a06835
    Yang Shi authored
    Other than munmap, mremap might be used to shrink memory mapping too.
    So, it may hold write mmap_sem for long time when shrinking large
    mapping, as what commit ("mm: mmap: zap pages with read mmap_sem in
    munmap") described.
    
    The mremap() will not manipulate vmas anymore after __do_munmap() call for
    the mapping shrink use case, so it is safe to downgrade to read mmap_sem.
    
    So, the same optimization, which downgrades mmap_sem to read for zapping
    pages, is also feasible and reasonable to this case.
    
    The period of holding exclusive mmap_sem for shrinking large mapping
    would be reduced significantly with this optimization.
    
    MREMAP_FIXED and MREMAP_MAYMOVE are more complicated to adopt this
    optimization since they need manipulate vmas after do_munmap(),
    downgrading mmap_sem may create race window.
    
    Simple mapping shrink is the low hanging fruit, and it may cover the
    most cases of unmap with munmap together.
    
    [akpm@linux-foundation.org: tweak comment]
    [yang.s...
    85a06835
mremap.c 17.1 KB