[096/165] ext4: Fix fencepost error in chosing choosing group vs file preallocation.

From: Greg KH
Date: Fri Jul 30 2010 - 14:58:26 EST

2.6.32-stable review patch. If anyone has any objections, please let us know.


commit cc483f102c3f703e853c96f95a654f0106fb2603 upstream (as of v2.6.33-git11)

The ext4 multiblock allocator decides whether to use group or file
preallocation based on the file size. When the file size reaches
s_mb_stream_request (default is 16 blocks), it changes to use a
file-specific preallocation. This is cool, but it has a tiny problem.

See a simple script:
mkfs.ext4 -b 1024 /dev/sda8 1000000
mount -t ext4 -o nodelalloc /dev/sda8 /mnt/ext4
cat /mnt/4096>>/mnt/ext4/a #4096 is a file with 4096 characters.
cat /mnt/4096>>/mnt/ext4/b
debuge4fs -R 'stat a' /dev/sda8|grep BLOCKS -A 1

And you get
(0-14):8705-8719, (15):2356, (16-19):8465-8468

So there are 3 extents, a bit strange for the lonely 15th logical
block. As we write to the 16 blocks, we choose file preallocation in
ext4_mb_group_or_file, but in ext4_mb_normalize_request, we meet with
the 16*1024 range, so no preallocation will be carried. file b then
reserves the space after '2356', so when when write 16, we start from
another part.

This patch just change the check in ext4_mb_group_or_file, so
that for the lonely 15 we will still use group preallocation.
After the patch, we will get:
debuge4fs -R 'stat a' /dev/sda8|grep BLOCKS -A 1
(0-15):8705-8720, (16-19):8465-8468

Looks more sane. Thanks.

Signed-off-by: Tao Ma <tao.ma@xxxxxxxxxx>
Signed-off-by: "Theodore Ts'o" <tytso@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>
fs/ext4/mballoc.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -3938,7 +3938,7 @@ static void ext4_mb_group_or_file(struct

/* don't use group allocation for large files */
size = max(size, isize);
- if (size >= sbi->s_mb_stream_request) {
+ if (size > sbi->s_mb_stream_request) {
ac->ac_flags |= EXT4_MB_STREAM_ALLOC;

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