mm, frontswap: convert frontswap_enabled to static key
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.cz
Signed-off-by: Vlastimil 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 031713ab..78cfa29 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -2493,7 +2493,7 @@
goto bad_swap;
}
/* frontswap enabled? set up bit-per-page map for frontswap */
- if (frontswap_enabled)
+ if (IS_ENABLED(CONFIG_FRONTSWAP))
frontswap_map = vzalloc(BITS_TO_LONGS(maxpages) * sizeof(long));
if (p->bdev &&(swap_flags & SWAP_FLAG_DISCARD) && swap_discardable(p)) {