On mon, 23 Sep 2013 11:35:11 +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 <fdman...@gmail.com>
Reviewed-by: Miao Xie <mi...@cn.fujitsu.com>
> ---
>
> V2: Use writeback_inodes_sb() instead of btrfs_start_all_delalloc_inodes(), as
> suggested by Miao Xie.
> V3: Use btrfs_start_all_delalloc_inodes() instead but outside btrfs_sync_fs(),
> in the sync IOCTL handler. Using writeback_inodes_sb() is not very honest
> because it doesn't guarantee inode data is persisted and we have no way
> to know if persistence really happened or not, returning 0 (success)
> always.
> Thanks Liu Bo for the suggestion.
> V4: Be even more honest in the sync IOCTL handler - don't always return
> success
> regardless of the result of the btrfs_sync_fs() call.
>
> fs/btrfs/ioctl.c | 12 +++++++++---
> 1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index 9d46f60..385c58f 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -4557,9 +4557,15 @@ long btrfs_ioctl(struct file *file, unsigned int
> return btrfs_ioctl_logical_to_ino(root, argp);
> case BTRFS_IOC_SPACE_INFO:
> return btrfs_ioctl_space_info(root, argp);
> - case BTRFS_IOC_SYNC:
> - btrfs_sync_fs(file->f_dentry->d_sb, 1);
> - return 0;
> + case BTRFS_IOC_SYNC: {
> + int ret;
> +
> + ret = btrfs_start_all_delalloc_inodes(root->fs_info, 0);
> + if (ret)
> + return ret;
> + ret = btrfs_sync_fs(file->f_dentry->d_sb, 1);
> + return ret;
> + }
> case BTRFS_IOC_START_SYNC:
> return btrfs_ioctl_start_sync(root, argp);
> case BTRFS_IOC_WAIT_SYNC:
>
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