RE: [RFC PATCH] fs: file freeze support

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

> On Mon 19-01-15 22:07:01, Namjae Jeon wrote:
> > > > When this state is set, any process which tries to modify the file's address
> > > > space, either by pagefault mmap writes or using write(2), will block until
> > > > the this state is cleared. I_WRITE_FREEZED is set by calling FS_IOC_FWFREEZE
> > > > ioctl and clear by FS_IOC_FWTHAW ioctl.
> > > >
> > > > File write freeze functionality, when used in conjunction with
> > > > inode's immutable flag can be used for creating truly stable file snapshots
> > > > wherein write freeze will prevent any modification to the file from already
> > > > open file descriptors and immutable flag will prevent any new modification
> > > > to the file. One of the intended uses for stable file snapshots would be in
> > > > the defragmentation applications which defrags single file.
> > >
> > > I don't quite understand why the full filesystem freeze is
> > > necessary? The thaw occurs immediately after I_WRITE_FREEZED is set,
> > We started by looking at fs freeze for file freeze implementation,
> > So got biased for using fs freeze or similar approach.
> > Thanks for suggesting a better way.
> >
> > > which means there's nothing that prevent the file from being
> > > truncated or otherwise modified by fallocate, etc while it is
> > > frozen....
> > Right, So, After that, we had also thought of setting immutable
> > flag of inode. Immutable flag + I_WRITE_FROZEN => truly frozen file.
> >
> > >
> > > AFAICT, fsync will bring the file down to a consistent state and
> > > we've already got freeze hooks for all inode modification
> > > operations. We also have IO barriers for truncate operations so that
> > > we can wait for all outstanding IO to complete, so I would have
> > > thought this covers all bases for an inode freeze. i.e.:
> > Right.
> >
> > >
> > > i_mutex -> I_FROZEN -> fsync -> inode_dio_wait
> > >
> > > Should give us a clean inode where there are not ongoing operations
> > > by the time that inode_dio_wait() completes. All new modification
> > > operations need to check I_FROZEN in addition to the superblock
> > > freeze checks...
> > I checked the routines where checks for I_FROZEN would be required.
> > Most of them are Ok but do_unlinkat() confuses me a little.
> > vfs_unlink is called under parent inode's i_mutex, so we cannot sleep
> > keeping parent's i_mutex held.
> > i.e while freezing file, all file in directory are blocked by parent
> > i_mutex. Is it ok to release parnets->mutex before checking for I_FROZEN
> > or there is some idea?
>   So I believe Dave thought that you'd just reuse places we currently use
> to call sb_start_write() / mnt_want_write(). You'd probably have to come up
> with a function like path_want_write() (takes struct path as an argument)
> and which will call mnt_want_write(), sb_start_write(), and do appropriate
> inode freeze handling. Then you replace all calls to mnt_want_write() with
> calls to path_want_write()... Possibly you can also provide a trivial
> wrapper for path_want_write() which takes struct file instead.
Okay, I will rework as your suggestion.
> 
> This should also deal with the locking problems you describe above as
> mnt_want_write() is always called before taking i_mutex.
Right. will check. I will back with V2 patch.

Thanks for review!
> 
> 								Honza
> --
> Jan Kara <jack@suse.cz>
> SUSE Labs, CR