mm: zero hash tables in allocator

Add a new flag HASH_ZERO which when provided grantees that the hash
table that is returned by alloc_large_system_hash() is zeroed.  In most
cases that is what is needed by the caller.  Use page level allocator's
__GFP_ZERO flags to zero the memory.  It is using memset() which is
efficient method to zero memory and is optimized for most platforms.

Link: http://lkml.kernel.org/r/1488432825-92126-3-git-send-email-pasha.tatashin@oracle.comSigned-off-by: NPavel Tatashin <pasha.tatashin@oracle.com>
Reviewed-by: NBabu Moger <babu.moger@oracle.com>
Cc: David Miller <davem@davemloft.net>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

include/linux/bootmem.h

@@ -358,6 +358,7 @@ extern void *alloc_large_system_hash(const char *tablename,
 #define HASH_EARLY	0x00000001	/* Allocating during early boot? */
 #define HASH_SMALL	0x00000002	/* sub-page allocation allowed, min
 					 * shift passed via *_hash_shift */
+#define HASH_ZERO	0x00000004	/* Zero allocated hash table */
 
 /* Only NUMA needs hash distribution. 64bit NUMA architectures have
  * sufficient vmalloc space.

mm/page_alloc.c

@@ -7198,6 +7198,7 @@ void *__init alloc_large_system_hash(const char *tablename,
 	unsigned long long max = high_limit;
 	unsigned long log2qty, size;
 	void *table = NULL;
+	gfp_t gfp_flags;
 
 	/* allow the kernel cmdline to have a say */
 	if (!numentries) {
@@ -7242,12 +7243,17 @@ void *__init alloc_large_system_hash(const char *tablename,
 
 	log2qty = ilog2(numentries);
 
+	/*
+	 * memblock allocator returns zeroed memory already, so HASH_ZERO is
+	 * currently not used when HASH_EARLY is specified.
+	 */
+	gfp_flags = (flags & HASH_ZERO) ? GFP_ATOMIC | __GFP_ZERO : GFP_ATOMIC;
 	do {
 		size = bucketsize << log2qty;
 		if (flags & HASH_EARLY)
 			table = memblock_virt_alloc_nopanic(size, 0);
 		else if (hashdist)
-			table = __vmalloc(size, GFP_ATOMIC, PAGE_KERNEL);
+			table = __vmalloc(size, gfp_flags, PAGE_KERNEL);
 		else {
 			/*
 			 * If bucketsize is not a power-of-two, we may free
@@ -7255,8 +7261,8 @@ void *__init alloc_large_system_hash(const char *tablename,
 			 * alloc_pages_exact() automatically does
 			 */
 			if (get_order(size) < MAX_ORDER) {
-				table = alloc_pages_exact(size, GFP_ATOMIC);
-				kmemleak_alloc(table, size, 1, GFP_ATOMIC);
+				table = alloc_pages_exact(size, gfp_flags);
+				kmemleak_alloc(table, size, 1, gfp_flags);
 			}
 		}
 	} while (!table && size > PAGE_SIZE && --log2qty);