Re: [PATCH 05/10 v5] ext4: lookup block mapping in extent status tree

[Zheng Liu <gnehzuil.liu@gmail.com>]

On Fri, Feb 15, 2013 at 12:25:49PM -0500, Theodore Ts'o wrote:
> On Fri, Feb 15, 2013 at 03:06:26PM +0800, Zheng Liu wrote:
> > 
> > I am planning to refine the get_block_t and *map_blocks functions.  At
> > that time I will try to fix this problem.
> 
> Note that get_block_t can't be changed without disrupting the Direct
> I/O functions which are generic VFS functions.  There's been talk of
> trying to clean up DIO, but it will probably require building a
> parallel infrastructure in the generic layer, and then transitioning
> individual file systems over to it.  It is definitely a mess, but it's
> going to be a very tricky problem.  I suspect we'll be talking about
> it at LSF/MM.
> 
> One thing thing which might be an interesting thing to do that
> wouldn't require wholesale changes to generic code would be to
> transition ext4_readpages() to use fs/ext4/page-io.c.  Not for this
> merge window, in all likelihood, but right now we are calling
> ext4_get_block() for every single page that we read in, while is
> wasteful.  It would be nice if ext4_readpages() called
> ext4_map_blocks() for each extent, and then submitted it using the
> page-io.c functions so we don't end up calling into ext4_map_blocks()
> quite as much.
> 
> That will ease our scalability and remove locking overhead, in
> addition to saving CPU for the buffered I/O readpages path.
> Eventually it would be good to do this for DIO as well, but that's
> going to require a lot more work, and coordination with the developers
> of btrfs, xfs, etc.

To be honest, my initial idea is only to split ext4_map_blocks into
ext4_map_blocks_read and ext4_map_blocks_write, and do some cleanups.
Thanks for your suggestions.  I will look at it carefully after the
patch series of extent status tree has been applied.

Thanks,
                                                - Zheng