• Vlastimil Babka's avatar
    mm, frontswap: convert frontswap_enabled to static key · 8ea1d2a1
    Vlastimil Babka authored
    I have noticed that frontswap.h first declares "frontswap_enabled" as
    extern bool variable, and then overrides it with "#define
    frontswap_enabled (1)" for CONFIG_FRONTSWAP=Y or (0) when disabled.  The
    bool variable isn't actually instantiated anywhere.
    
    This all looks like an unfinished attempt to make frontswap_enabled
    reflect whether a backend is instantiated.  But in the current state,
    all frontswap hooks call unconditionally into frontswap.c just to check
    if frontswap_ops is non-NULL.  This should at least be checked inline,
    but we can further eliminate the overhead when CONFIG_FRONTSWAP is
    enabled and no backend registered, using a static key that is initially
    disabled, and gets enabled only upon first backend registration.
    
    Thus, checks for "frontswap_enabled" are replaced with
    "frontswap_enabled()" wrapping the static key check.  There are two
    exceptions:
    
    - xen's selfballoon_process() was testing frontswap_enabled in code guarded
      by #ifdef CONFIG_FRONTSWAP, which was effectively always true when reachable.
      The patch just removes this check. Using frontswap_enabled() does not sound
      correct here, as this can be true even without xen's own backend being
      registered.
    
    - in SYSCALL_DEFINE2(swapon), change the check to IS_ENABLED(CONFIG_FRONTSWAP)
      as it seems the bitmap allocation cannot currently be postponed until a
      backend is registered. This means that frontswap will still have some
      memory overhead by being configured, but without a backend.
    
    After the patch, we can expect that some functions in frontswap.c are
    called only when frontswap_ops is non-NULL.  Change the checks there to
    VM_BUG_ONs.  While at it, convert other BUG_ONs to VM_BUG_ONs as
    frontswap has been stable for some time.
    
    [akpm@linux-foundation.org: coding-style fixes]
    Link: http://lkml.kernel.org/r/1463152235-9717-1-git-send-email-vbabka@suse.czSigned-off-by: 's avatarVlastimil Babka <vbabka@suse.cz>
    Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
    Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
    Cc: David Vrabel <david.vrabel@citrix.com>
    Cc: Juergen Gross <jgross@suse.com>
    Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
    Signed-off-by: 's avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: 's avatarLinus Torvalds <torvalds@linux-foundation.org>
    8ea1d2a1
frontswap.c 14.3 KB