Executing an fsync() on a file-descriptor of a partition flushes the
caches for that partition by calling blkdev_fsync(). However, it seems
that reading data through the parent device will still return the old
cached data.
The problem can be worked around by forcing the caches to be flushed
with either
# blockdev --flushbufs ${dev_disk}
or
# echo 3 > /proc/sys/vm/drop_caches
One of the use-cases that shows this problem:
1) create two or more partitions on a device
- use fdisk to create /dev/sdb1 and /dev/sdb2
2) format and mount one of the partition
- mkfs -t ext3 /dev/sdb1
3) read through the main device to have something in the cache
- read /dev/sdb with dd or use something like "parted /dev/sdb print"
4) now write something to /dev/sdb2, format the partition for example
- mkfs -t ext3 /dev/sdb2
5) read the blocks where sdb2 starts, through /dev/sdb
- use dd or do again a "parted /dev/sdb print"
The cache for the block-device is not synced if the block-device is kept
open (due to a mounted partition, for example). Only when all users for
the disk have exited, the cache for the disk is made consistent again.
Without this patch, calling "blockdev --flushbufs" or dropping the
caches, the result in 5) is the same as in 3). Reading the same area
through /dev/sdb2 shows the inconsistancy between the two caches.
CC: Bryn M. Reeves <bmr@xxxxxxxxxx>
Acked-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Reviewed-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
Signed-off-by: Niels de Vos <ndevos@xxxxxxxxxx>
---
v3:
- Correct commit message, no need to mention blkdev_issue_flush().
- Include a use-case for giving a clearer understaning of the problem
that is being addressed.
- Include received Acked-by and Reviewed-by, only the commit message
changed in this v3.
v2:
- Do not call invalidate_bdev() from blkdev_issue_flush() and prevent
performance degration with journalled filesystems.
Suggested was to call invalidate_bdev() in fsync_bdev(), but this is
not in the call-path of mkfs.ext3 and similar tools. Hence the issue
persists.
- Correct phrasing a little, changing ioctl-BLKFLSBUF is not required.
- This issue also occurs when doing an ioctl-BLKFLSBUF on a partition.
Reading the whole disk will still return cached data. If this is an
issue, it will need a seperate patch.
---
fs/block_dev.c | 4 ++++
1 files changed, 4 insertions(+), 0 deletions(-)
diff --git a/fs/block_dev.c b/fs/block_dev.c
index 0e575d1..433c4de 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -424,6 +424,10 @@ int blkdev_fsync(struct file *filp, loff_t start, loff_t end, int datasync)
if (error == -EOPNOTSUPP)
error = 0;
+ /* invalidate parent block_device */
+ if (!error && bdev != bdev->bd_contains)
+ invalidate_bdev(bdev->bd_contains);
+
return error;
}
EXPORT_SYMBOL(blkdev_fsync);
--
1.7.6.5
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
