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Subject[PATCH v2] fs: Invalidate the cache for a parent block-device if fsync() is called for a partition
Date
Executing an fsync() on a file-descriptor of a partition flushes the
caches for that partition by calling blkdev_issue_flush(). However, it
seems that reading data through the parent device will still return the
old cached data.

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.

Calling invalidate_bdev() on the parent block-device in case
blkdev_fsync() was called for a partition, fixes this.

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
CC: Bryn M. Reeves <bmr@redhat.com>
CC: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Niels de Vos <ndevos@redhat.com>

---
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


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