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    SubjectRe: [PATCH v2] fs: Invalidate the cache for a parent block-device if fsync() is called for a partition
    I suggest a viro cc on this one.

    On Mon, 23 Jan 2012 10:38:29 +0000 Niels de Vos <ndevos@redhat.com> wrote:

    > Executing an fsync() on a file-descriptor of a partition flushes the
    > caches for that partition by calling blkdev_issue_flush(). However, it

    The changelog is stale.

    > 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

    Please include your (useful) problem description in the changelog:

    : The problem that was noticed is the following:
    : 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"
    :
    : 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.
    :
    : With this patch, or one of the workarounds, the data read through
    : /dev/sdb and /dev/sdb2 is the same.

    >
    > ...
    >
    > --- 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;
    > }

    It doesn't seem terribly logical to do this in blkdev_fsync(). Why not
    do it right there in blkdev_ioctl()'s "case BLKFLSBUF"?

    Bear in mind that invalidate_bdev() isn't a very strong function -
    it won't drop pages which are dirty or under writeback nor pages which
    others have a reference on. But I can see that this change would be a
    best-effort user-convenience thing.



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