From f203c3b33f0891da98ae3dcf829851c48473ed60 Mon Sep 17 00:00:00 2001 From: Chao Yu Date: Thu, 9 Oct 2014 15:30:04 -0700 Subject: [PATCH] zbud: avoid accessing last unused freelist For now, there are NCHUNKS of 64 freelists in zbud_pool, the last unbuddied[63] freelist linked with all zbud pages which have free chunks of 63. Calculating according to context of num_free_chunks(), our max chunk number of unbuddied zbud page is 62, so none of zbud pages will be added/removed in last freelist, but still we will try to find an unbuddied zbud page in the last unused freelist, it is unneeded. This patch redefines NCHUNKS to 63 as free chunk number in one zbud page, hence we can decrease size of zpool and avoid accessing the last unused freelist whenever failing to allocate zbud from freelist in zbud_alloc. Signed-off-by: Chao Yu Cc: Seth Jennings Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/zbud.c | 13 +++++++------ 1 file changed, 7 insertions(+), 6 deletions(-) diff --git a/mm/zbud.c b/mm/zbud.c index f26e7fcc7fa2..ecf1dbef6983 100644 --- a/mm/zbud.c +++ b/mm/zbud.c @@ -60,15 +60,17 @@ * NCHUNKS_ORDER determines the internal allocation granularity, effectively * adjusting internal fragmentation. It also determines the number of * freelists maintained in each pool. NCHUNKS_ORDER of 6 means that the - * allocation granularity will be in chunks of size PAGE_SIZE/64, and there - * will be 64 freelists per pool. + * allocation granularity will be in chunks of size PAGE_SIZE/64. As one chunk + * in allocated page is occupied by zbud header, NCHUNKS will be calculated to + * 63 which shows the max number of free chunks in zbud page, also there will be + * 63 freelists per pool. */ #define NCHUNKS_ORDER 6 #define CHUNK_SHIFT (PAGE_SHIFT - NCHUNKS_ORDER) #define CHUNK_SIZE (1 << CHUNK_SHIFT) -#define NCHUNKS (PAGE_SIZE >> CHUNK_SHIFT) #define ZHDR_SIZE_ALIGNED CHUNK_SIZE +#define NCHUNKS ((PAGE_SIZE - ZHDR_SIZE_ALIGNED) >> CHUNK_SHIFT) /** * struct zbud_pool - stores metadata for each zbud pool @@ -268,10 +270,9 @@ static int num_free_chunks(struct zbud_header *zhdr) { /* * Rather than branch for different situations, just use the fact that - * free buddies have a length of zero to simplify everything. -1 at the - * end for the zbud header. + * free buddies have a length of zero to simplify everything. */ - return NCHUNKS - zhdr->first_chunks - zhdr->last_chunks - 1; + return NCHUNKS - zhdr->first_chunks - zhdr->last_chunks; } /***************** -- 2.30.2