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[PATCH v2] xfs: Fix overallocation in xfs_buf_allocate_memory()

To: xfs@xxxxxxxxxxx
Subject: [PATCH v2] xfs: Fix overallocation in xfs_buf_allocate_memory()
From: Jan Kara <jack@xxxxxxx>
Date: Wed, 6 Jun 2012 00:32:26 +0200
Cc: Ben Myers <bpm@xxxxxxx>, Alex Elder <elder@xxxxxxxxxx>, Jan Kara <jack@xxxxxxx>, David Chinner <dchinner@xxxxxxxxxx>
Commit de1cbee which removed b_file_offset in favor of b_bn introduced a bug
causing xfs_buf_allocate_memory() 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 is unnecessary.

Dave says xfs_buf_alloc() doesn't need to set b_bn to -1 anymore since the
buffer is inserted into the cache only after being fully initialized now.
So just make xfs_buf_alloc() fill in proper block number from the beginning.

CC: David Chinner <dchinner@xxxxxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
---
 fs/xfs/xfs_buf.c |   16 ++--------------
 1 files changed, 2 insertions(+), 14 deletions(-)

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index 172d3cc..a4beb42 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -201,14 +201,7 @@ xfs_buf_alloc(
        bp->b_length = numblks;
        bp->b_io_length = numblks;
        bp->b_flags = flags;
-
-       /*
-        * We do not set the block number here in the buffer because we have not
-        * finished initialising the buffer. We insert the buffer into the cache
-        * in this state, so this ensures that we are unable to do IO on a
-        * buffer that hasn't been fully initialised.
-        */
-       bp->b_bn = XFS_BUF_DADDR_NULL;
+       bp->b_bn = blkno;
        atomic_set(&bp->b_pin_count, 0);
        init_waitqueue_head(&bp->b_waiters);
 
@@ -567,11 +560,6 @@ xfs_buf_get(
        if (bp != new_bp)
                xfs_buf_free(new_bp);
 
-       /*
-        * Now we have a workable buffer, fill in the block number so
-        * that we can do IO on it.
-        */
-       bp->b_bn = blkno;
        bp->b_io_length = bp->b_length;
 
 found:
@@ -772,7 +760,7 @@ xfs_buf_get_uncached(
        int                     error, i;
        xfs_buf_t               *bp;
 
-       bp = xfs_buf_alloc(target, 0, numblks, 0);
+       bp = xfs_buf_alloc(target, XFS_BUF_DADDR_NULL, numblks, 0);
        if (unlikely(bp == NULL))
                goto fail;
 
-- 
1.7.1

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