• David Howells's avatar
    nommu: make the initial mmap allocation excess behaviour Kconfig configurable · fc4d5c29
    David Howells authored
    NOMMU mmap() has an option controlled by a sysctl variable that determines
    whether the allocations made by do_mmap_private() should have the excess
    space trimmed off and returned to the allocator.  Make the initial setting
    of this variable a Kconfig configuration option.
    
    The reason there can be excess space is that the allocator only allocates
    in power-of-2 size chunks, but mmap()'s can be made in sizes that aren't a
    power of 2.
    
    There are two alternatives:
    
     (1) Keep the excess as dead space.  The dead space then remains unused for the
         lifetime of the mapping.  Mappings of shared objects such as libc, ld.so
         or busybox's text segment may retain their dead space forever.
    
     (2) Return the excess to the allocator.  This means that the dead space is
         limited to less than a page per mapping, but it means that for a transient
         process, there's more chance of fragmentation as the excess space may be
         reused fairly quickly.
    
    During the boot process, a lot of transient processes are created, and
    this can cause a lot of fragmentation as the pagecache and various slabs
    grow greatly during this time.
    
    By turning off the trimming of excess space during boot and disabling
    batching of frees, Coldfire can manage to boot.
    
    A better way of doing things might be to have /sbin/init turn this option
    off.  By that point libc, ld.so and init - which are all long-duration
    processes - have all been loaded and trimmed.
    Reported-by: default avatarLanttor Guo <lanttor.guo@freescale.com>
    Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
    Tested-by: default avatarLanttor Guo <lanttor.guo@freescale.com>
    Cc: Greg Ungerer <gerg@snapgear.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    fc4d5c29
Kconfig 7.86 KB