mm: Kconfig: Add support for config size of purging vmap_area

This size is the maximum amount of virtual address space we gather
up before attempting to purge with a TLB flush. It is 128M in most cases.
With repeated and high size vmalloc operations, it may easily generate
more fragments. This is wasting limited vmalloc area, for 32bits.
So make it configable and the default multiplier as 8, 32bits only.

Change-Id: I68a75acb16d3cff05f8b13c05ae78922269e219f
Signed-off-by: Zhenhua Huang <zhenhuah@codeaurora.org>

mm/Kconfig

@@ -628,6 +628,16 @@ config ZSMALLOC_STAT
 	  information to userspace via debugfs.
 	  If unsure, say N.
 
+config VMAP_LAZY_PURGING_FACTOR
+	int "multiplier to the size of purged vmap areas"
+	default "8" if ARM
+	default "32"
+	help
+	  It is used as a multiplier to the max VA pages purged in a
+	  single attempt. For 32-bit in order to reduce fragmentation
+	  of vmalloc space, we decrease the default value to "8".
+
+
 config GENERIC_EARLY_IOREMAP
 	bool
 

mm/vmalloc.c

@@ -1260,7 +1260,8 @@ static unsigned long lazy_max_pages(void)
 
 	log = fls(num_online_cpus());
 
-	return log * (32UL * 1024 * 1024 / PAGE_SIZE);
+	return log * (1UL * CONFIG_VMAP_LAZY_PURGING_FACTOR *
+					1024 * 1024 / PAGE_SIZE);
 }
 
 static atomic_long_t vmap_lazy_nr = ATOMIC_LONG_INIT(0);