[PATCH 1.5/2 v2] dm stripe: optimize sector division

[Mike Snitzer <snitzer@redhat.com>]

From: Mikulas Patocka <mpatocka@redhat.com>

Optimize sector division --- if the number of stripes is a power of two,
we don't have to do expensive division and can do shift and mask instead.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 drivers/md/dm-stripe.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

v2: minor code cleanup in stripe_map_sector

Index: linux-2.6-block/drivers/md/dm-stripe.c
===================================================================
--- linux-2.6-block.orig/drivers/md/dm-stripe.c
+++ linux-2.6-block/drivers/md/dm-stripe.c
@@ -25,6 +25,7 @@ struct stripe {
 
 struct stripe_c {
 	uint32_t stripes;
+	int stripes_shift;
 
 	/* The size of this target / num. stripes */
 	sector_t stripe_width;
@@ -164,6 +165,10 @@ static int stripe_ctr(struct dm_target *
 	sc->ti = ti;
 
 	sc->stripes = stripes;
+	if (!(stripes & (stripes - 1)))
+		sc->stripes_shift = ffs(stripes) - 1;
+	else
+		sc->stripes_shift = -1;
 	sc->stripe_width = width;
 	ti->split_io = chunk_size;
 	ti->num_flush_requests = stripes;
@@ -213,7 +218,12 @@ static void stripe_map_sector(struct str
 {
 	sector_t offset = sector - sc->ti->begin;
 	sector_t chunk = offset >> sc->chunk_shift;
-	*stripe = sector_div(chunk, sc->stripes);
+
+	if (likely(sc->stripes_shift >= 0)) {
+		*stripe = chunk & ((1 << sc->stripes_shift) - 1);
+		chunk >>= sc->stripes_shift;
+	} else
+		*stripe = sector_div(chunk, sc->stripes);
 	*result = (chunk << sc->chunk_shift) | (offset & sc->chunk_mask);
 }