On Mon, Sep 23, 2013 at 10:23 AM, Filipe David Borba Manana
<fdman...@gmail.com> 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 <fdman...@gmail.com>
> ---
>
> V2: Use writeback_inodes_sb() instead of btrfs_start_all_delalloc_inodes(), as
> suggested by Miao Xie.
>
> fs/btrfs/super.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index 6ab0df5..38b4392 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -921,6 +921,7 @@ int btrfs_sync_fs(struct super_block *sb, int wait)
> return 0;
> }
>
> + writeback_inodes_sb(sb, WB_REASON_SYNC);
> btrfs_wait_all_ordered_extents(fs_info);
Ignore this 2nd patch version please, for 2 reasons:
1) It triggers a WARN_ON because writeback_inodes_sb() requires the
sb->u_mount semaphore to be acquired before, which is not always the
case (it is when called through btrfs_kill_super, otherwise it isn't)
2) It doesn't guarantee that inodes are actually written (see comment
of writeback_inodes_sb()), so we can return 0 (success) when the
writes actually didn't happen/succeed. Because of this,
btrfs_start_all_delalloc_inodes() is more honest.
>
> trans = btrfs_attach_transaction_barrier(root);
> --
> 1.7.9.5
>
--
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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