• Andrzej Hajda's avatar
    mm/util: add kstrdup_const · a4bb1e43
    Andrzej Hajda authored
    
    
    kstrdup() is often used to duplicate strings where neither source neither
    destination will be ever modified.  In such case we can just reuse the
    source instead of duplicating it.  The problem is that we must be sure
    that the source is non-modifiable and its life-time is long enough.
    
    I suspect the good candidates for such strings are strings located in
    kernel .rodata section, they cannot be modifed because the section is
    read-only and their life-time is equal to kernel life-time.
    
    This small patchset proposes alternative version of kstrdup -
    kstrdup_const, which returns source string if it is located in .rodata
    otherwise it fallbacks to kstrdup.  To verify if the source is in
    .rodata function checks if the address is between sentinels
    __start_rodata, __end_rodata.  I guess it should work with all
    architectures.
    
    The main patch is accompanied by four patches constifying kstrdup for
    cases where situtation described above happens frequently.
    
    I have tested the patchset on mobile platform (exynos4210-trats) and it
    saves 3272 string allocations.  Since minimal allocation is 32 or 64
    bytes depending on Kconfig options the patchset saves respectively about
    100KB or 200KB of memory.
    
    Stats from tested platform show that the main offender is sysfs:
    
    By caller:
      2260 __kernfs_new_node
        631 clk_register+0xc8/0x1b8
        318 clk_register+0x34/0x1b8
          51 kmem_cache_create
          12 alloc_vfsmnt
    
    By string (with count >= 5):
        883 power
        876 subsystem
        135 parameters
        132 device
         61 iommu_group
        ...
    
    This patch (of 5):
    
    Add an alternative version of kstrdup which returns pointer to constant
    char array.  The function checks if input string is in persistent and
    read-only memory section, if yes it returns the input string, otherwise it
    fallbacks to kstrdup.
    
    kstrdup_const is accompanied by kfree_const performing conditional memory
    deallocation of the string.
    Signed-off-by: default avatarAndrzej Hajda <a.hajda@samsung.com>
    Cc: Marek Szyprowski <m.szyprowski@samsung.com>
    Cc: Kyungmin Park <kyungmin.park@samsung.com>
    Cc: Mike Turquette <mturquette@linaro.org>
    Cc: Alexander Viro <viro@zeniv.linux.org.uk>
    Cc: Christoph Lameter <cl@linux.com>
    Cc: Pekka Enberg <penberg@kernel.org>
    Cc: David Rientjes <rientjes@google.com>
    Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
    Cc: Tejun Heo <tj@kernel.org>
    Cc: Greg KH <greg@kroah.com>
    Cc: Geert Uytterhoeven <geert@linux-m68k.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    a4bb1e43
util.c 10.3 KB