[PATCH] xfs: Fix overallocation in xfs_buf_allocate_memory()

[Jan Kara]

Commit 0e6e847f which introduced xfs_buf_allocate_memory() function has a bug
causing the function to overestimate the number of necessary pages. The problem
is that xfs_buf_alloc() sets b_bn to -1 and thus effectively every buffer is
straddling a page boundary which causes xfs_buf_allocate_memory() to allocate
two pages and use vmalloc() for access which slows things down.

Fix the code to use correct block number.

Signed-off-by: Jan Kara <jack at suse.cz>
---
 fs/xfs/xfs_buf.c |    7 ++++---
 1 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index 172d3cc..b67cc83 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -296,6 +296,7 @@ xfs_buf_free(
 STATIC int
 xfs_buf_allocate_memory(
 	xfs_buf_t		*bp,
+	xfs_daddr_t		blkno,
 	uint			flags)
 {
 	size_t			size;
@@ -334,8 +335,8 @@ xfs_buf_allocate_memory(
 	}
 
 use_alloc_page:
-	start = BBTOB(bp->b_bn) >> PAGE_SHIFT;
-	end = (BBTOB(bp->b_bn + bp->b_length) + PAGE_SIZE - 1) >> PAGE_SHIFT;
+	start = BBTOB(blkno) >> PAGE_SHIFT;
+	end = (BBTOB(blkno + bp->b_length) + PAGE_SIZE - 1) >> PAGE_SHIFT;
 	page_count = end - start;
 	error = _xfs_buf_get_pages(bp, page_count, flags);
 	if (unlikely(error))
@@ -552,7 +553,7 @@ xfs_buf_get(
 	if (unlikely(!new_bp))
 		return NULL;
 
-	error = xfs_buf_allocate_memory(new_bp, flags);
+	error = xfs_buf_allocate_memory(new_bp, blkno, flags);
 	if (error) {
 		kmem_zone_free(xfs_buf_zone, new_bp);
 		return NULL;
-- 
1.7.1