On Wed, Jan 6, 2016 at 11:00 AM, Zhao Lei <zhaolei@xxxxxxxxxxxxxx> wrote:
> btrfs failed in xfstests btrfs/080 with -o nodatacow.
>
> Can be reproduced by following script:
> DEV=/dev/vdg
> MNT=/mnt/tmp
>
> umount $DEV &>/dev/null
> mkfs.btrfs -f $DEV
> mount -o nodatacow $DEV $MNT
>
> dd if=/dev/zero of=$MNT/test bs=1 count=2048 &
> btrfs subvolume snapshot -r $MNT $MNT/test_snap &
> wait
> --
> We can see dd failed on NO_SPACE.
>
> Reason:
> __btrfs_buffered_write should run cow write when no_cow impossible,
> and current code is designed with above logic.
> But check_can_nocow() have 2 type of return value(0 and <0) on
> can_not_no_cow, and current code only continue write on first case,
> the second case happened in doing subvolume.
>
> Fix:
> Continue write when check_can_nocow() return 0 and <0.
>
> Signed-off-by: Zhao Lei <zhaolei@xxxxxxxxxxxxxx>
Reviewed-by: Filipe Manana <fdmanana@xxxxxxxx>
Could you please submit a test case for xfstests as well?
Thanks.
> ---
> fs/btrfs/file.c | 37 +++++++++++++++++--------------------
> 1 file changed, 17 insertions(+), 20 deletions(-)
>
> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> index 977e715..11fd981 100644
> --- a/fs/btrfs/file.c
> +++ b/fs/btrfs/file.c
> @@ -1516,27 +1516,24 @@ static noinline ssize_t __btrfs_buffered_write(struct file *file,
>
> reserve_bytes = num_pages << PAGE_CACHE_SHIFT;
>
> - if (BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW |
> - BTRFS_INODE_PREALLOC)) {
> - ret = check_can_nocow(inode, pos, &write_bytes);
> - if (ret < 0)
> - break;
> - if (ret > 0) {
> - /*
> - * For nodata cow case, no need to reserve
> - * data space.
> - */
> - only_release_metadata = true;
> - /*
> - * our prealloc extent may be smaller than
> - * write_bytes, so scale down.
> - */
> - num_pages = DIV_ROUND_UP(write_bytes + offset,
> - PAGE_CACHE_SIZE);
> - reserve_bytes = num_pages << PAGE_CACHE_SHIFT;
> - goto reserve_metadata;
> - }
> + if ((BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW |
> + BTRFS_INODE_PREALLOC)) &&
> + check_can_nocow(inode, pos, &write_bytes) > 0) {
> + /*
> + * For nodata cow case, no need to reserve
> + * data space.
> + */
> + only_release_metadata = true;
> + /*
> + * our prealloc extent may be smaller than
> + * write_bytes, so scale down.
> + */
> + num_pages = DIV_ROUND_UP(write_bytes + offset,
> + PAGE_CACHE_SIZE);
> + reserve_bytes = num_pages << PAGE_CACHE_SHIFT;
> + goto reserve_metadata;
> }
> +
> ret = btrfs_check_data_free_space(inode, pos, write_bytes);
> if (ret < 0)
> break;
> --
> 1.8.5.1
>
>
>
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--
Filipe David Manana,
"Reasonable men adapt themselves to the world.
Unreasonable men adapt the world to themselves.
That's why all progress depends on unreasonable men."
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