RE: [PATCH 3/3] ext4: Add support IOC_MOV_DATA ioctl

[Namjae Jeon <namjae.jeon@samsung.com>]

> On Tue, 8 Jul 2014 16:02:28 +0200 (CEST), Lukáš Czerner <lczerner@redhat.com> wrote:
> Non-text part: MULTIPART/MIXED
> > On Tue, 8 Jul 2014, Namjae Jeon wrote:
> >
> > > Date: Tue, 08 Jul 2014 21:00:02 +0900
> > > From: Namjae Jeon <namjae.jeon@samsung.com>
> > > To: Dave Chinner <david@fromorbit.com>, Theodore Ts'o <tytso@mit.edu>
> > > Cc: linux-ext4 <linux-ext4@vger.kernel.org>, linux-fsdevel@vger.kernel.org,
> > >     linux-kernel@vger.kernel.org, Lukáš Czerner <lczerner@redhat.com>,
> > >     Brian Foster <bfoster@redhat.com>, Christoph Hellwig <hch@infradead.org>,
> > >     Ashish Sangwan <a.sangwan@samsung.com>, xfs@oss.sgi.com
> > > Subject: [PATCH 3/3] ext4: Add support IOC_MOV_DATA ioctl
> > >
> > > This patch implements fs ioctl's IOC_MOV_DATA for Ext4.
> >
> > Hmm isn't this basically what ext4_move_extents() does ? eg.
> > EXT4_IOC_MOVE_EXT ?
> >
> > I guess that the intention here is to do the move, without actually
> > moving the data right ? But nevertheless maybe some code can be
> > shared with ext4_move_extents() ?
> It definitely can be shared, because it has specific case for unwritten
> data see move_extent_per_page().
If I understand correctly, mov_extent_per_page calls mext_replace_branches to
_replace_ extents from 1 inode to other inode. Please correct me if I am wrong.
ioc_mov_data will not replace extents, but it will actually _move_ extents into
hole from donor to receiver, leaving a hole at the place from where extents are
moved.
Could you elaborate more how it can be shared for unwritten data case ?

> But I think we can observe another way to unify this two things.
> An idea inspired by the fact that ioc_move_data works only for
> regular inodes, where orig_offset == donor_offset. 
Could you elaborate this point? 

> This is showstopper
> for  my utility e4defrag2 ( new version of e4defrag which is able defragment
> pack small files as described here :
> http://lists.openwall.net/linux-ext4/2014/04/28/3)
> 
> Proposed API is very similar to ext4_ext_migrate:
> Args:
>   orig_file: inode which we want to defragment
>   donor_file: a file which will be used as a donor of blocks
> 1) fallocate big donor_file
> 2) a) Create tmp inode wich nlink = 0
>    b) move extents required extents from  donor to tmp_donor_inode
>    c) return file descriptor (tmp_fd) to that tmp_donor_inode
> 4) Mark orig_file's inode with EXT4_STATE_EXT_MIGRATE state
> 5) Copy data from orig_file to tmp_fd
> 6) IOC_SWAP_EX: atomically swap  orig_file->i_data and tmp_fd->i_data
>    if EXT4_STATE_EXT_MIGRATE was not cleared.
> 
> This approach can works not only for regular file w/o journaling
> enabled, but also for journaled ones, and directories.
> 
> 
> 
> 
> >
> > -Lukas
> >

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