From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from aserp1040.oracle.com ([141.146.126.69]:41513 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753665Ab3IWJ7W (ORCPT ); Mon, 23 Sep 2013 05:59:22 -0400 Date: Mon, 23 Sep 2013 17:59:14 +0800 From: Liu Bo To: Filipe David Manana Cc: "linux-btrfs@vger.kernel.org" Subject: Re: [PATCH v2] Btrfs: fix sync fs to actually wait for all data to be persisted Message-ID: <20130923095913.GB18072@localhost.localdomain> Reply-To: bo.li.liu@oracle.com References: <1379883353-7358-1-git-send-email-fdmanana@gmail.com> <1379928200-21566-1-git-send-email-fdmanana@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: Sender: linux-btrfs-owner@vger.kernel.org List-ID: On Mon, Sep 23, 2013 at 10:53:20AM +0100, Filipe David Manana wrote: > On Mon, Sep 23, 2013 at 10:23 AM, 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 > > --- > > > > 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. What about case BTRFS_IOC_SYNC: btrfs_start_all_delalloc_inodes(); btrfs_sync_fs(file->f_dentry->d_sb, 1); return 0; This way, there is no impact on calling sync(1). -liubo