On sun, 22 Sep 2013 21:55:53 +0100, Filipe David Borba Manana wrote:
> Currently the fs sync function (super.c:btrfs_sync_fs()) doesn't
> wait for delayed work to finish before returning success to the
> caller. This change fixes this, ensuring that there's no data loss
> if a power failure happens right after fs sync returns success to
> the caller and before the next commit happens.
>
> Steps to reproduce the data loss issue:
>
> $ mkfs.btrfs -f /dev/sdb3
> $ mount /dev/sdb3 /mnt/btrfs
> $ perl -e '$d = ("\x41" x 6001); open($f,">","/mnt/btrfs/foobar"); print $f $d; close($f);' && btrfs fi sync /mnt/btrfs
>
> Right after the btrfs fi sync command (a second or 2 for example), power
> off the machine and reboot it. The file will be empty, as it can be verified
> after mounting the filesystem and through btrfs-debug-tree:
>
> $ btrfs-debug-tree /dev/sdb3 | egrep '\(257 INODE_ITEM 0\) itemoff' -B 3 -A 8
>
> item 3 key (256 DIR_INDEX 2) itemoff 3751 itemsize 36
> location key (257 INODE_ITEM 0) type FILE
> namelen 6 datalen 0 name: foobar
> item 4 key (257 INODE_ITEM 0) itemoff 3591 itemsize 160
> inode generation 7 transid 7 size 0 block group 0 mode 100644 links 1
> item 5 key (257 INODE_REF 256) itemoff 3575 itemsize 16
> inode ref index 2 namelen 6 name: foobar
> checksum tree key (CSUM_TREE ROOT_ITEM 0)
> leaf 29429760 items 0 free space 3995 generation 7 owner 7
> fs uuid 6192815c-af2a-4b75-b3db-a959ffb6166e
> chunk uuid b529c44b-938c-4d3d-910a-013b4700bcae
> uuid tree key (UUID_TREE ROOT_ITEM 0)
>
> After this patch, the data loss no longer happens after a power failure and
> btrfs-debug-tree shows:
>
> $ btrfs-debug-tree /dev/sdb3 | egrep '\(257 INODE_ITEM 0\) itemoff' -B 3 -A 8
> item 3 key (256 DIR_INDEX 2) itemoff 3751 itemsize 36
> location key (257 INODE_ITEM 0) type FILE
> namelen 6 datalen 0 name: foobar
> item 4 key (257 INODE_ITEM 0) itemoff 3591 itemsize 160
> inode generation 6 transid 6 size 6001 block group 0 mode 100644 links 1
> item 5 key (257 INODE_REF 256) itemoff 3575 itemsize 16
> inode ref index 2 namelen 6 name: foobar
> item 6 key (257 EXTENT_DATA 0) itemoff 3522 itemsize 53
> extent data disk byte 12845056 nr 8192
> extent data offset 0 nr 8192 ram 8192
> extent compression 0
> checksum tree key (CSUM_TREE ROOT_ITEM 0)
>
> Signed-off-by: Filipe David Borba Manana <fdmanana@xxxxxxxxx>
> ---
> fs/btrfs/super.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index 6ab0df5..557e38f 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -913,6 +913,7 @@ int btrfs_sync_fs(struct super_block *sb, int wait)
> struct btrfs_trans_handle *trans;
> struct btrfs_fs_info *fs_info = btrfs_sb(sb);
> struct btrfs_root *root = fs_info->tree_root;
> + int ret;
>
> trace_btrfs_sync_fs(wait);
>
> @@ -921,6 +922,10 @@ int btrfs_sync_fs(struct super_block *sb, int wait)
> return 0;
> }
>
> + ret = btrfs_start_all_delalloc_inodes(fs_info, 0);
> + if (ret)
> + return ret;
> +
I don't think we should call btrfs_start_all_delalloc_inodes(), because this function is also
called by do_sync(), but do_sync() syncs the whole fs before calling it, so if we add
btrfs_start_all_delalloc_inodes() here, we will sync the fs twice, and the second one is unnecessary.
Calling writeback_inodes_sb() before btrfs_sync_fs() is better way to fix this problem.
Thanks
Miao
> btrfs_wait_all_ordered_extents(fs_info);
>
> trans = btrfs_attach_transaction_barrier(root);
>
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