• Gioh Kim's avatar
    mm/vmalloc.c: enhance vm_map_ram() comment · 36437638
    Gioh Kim authored
    vm_map_ram() has a fragmentation problem when it cannot purge a
    chunk(ie, 4M address space) if there is a pinning object in that
    addresss space.  So it could consume all VMALLOC address space easily.
    We can fix the fragmentation problem by using vmap instead of
    vm_map_ram() but vmap() is known to be slow compared to vm_map_ram().
    Minchan said vm_map_ram is 5 times faster than vmap in his tests.  So I
    thought we should fix fragment problem of vm_map_ram because our
    proprietary GPU driver has used it heavily.
    On second thought, it's not an easy because we should reuse freed space
    for solving the problem and it could make more IPI and bitmap operation
    for searching hole.  It could mitigate API's goal which is very fast
    mapping.  And even fragmentation problem wouldn't show in 64 bit
    Another option is that the user should separate long-life and short-life
    object and use vmap for long-life but vm_map_ram for short-life.  If we
    inform the user about the characteristic of vm_map_ram the user can
    choose one according to the page lifetime.
    Let's add some notice messages to user.
    [akpm@linux-foundation.org: tweak comment text]
    Signed-off-by: default avatarGioh Kim <gioh.kim@lge.com>
    Reviewed-by: default avatarZhang Yanfei <zhangyanfei@cn.fujitsu.com>
    Cc: Minchan Kim <minchan@kernel.org>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
vmalloc.c 68.3 KB