Re: [PATCH] ext4: fix FS_IOC_GETFSMAP handling

["Theodore Ts'o" <tytso@mit.edu>]

On Fri, Oct 25, 2024 at 03:06:07PM +0800, Zhang Yi wrote:
> 
> Now it seems to be able to handle all the necessary metadata in the
> query range here, can we remove the processing of info->gfi_meta_list
> in ext4_getfsmap_datadev_helper() as well?

Not without further code refactoring; we still need it if there are
metadata blocks in the middle of the block group.  My main emphasis
was keeping the code changes as simple so it would be easy to
backport, and so I didn't do further optimizations.

As a related observation, I'm not entirely sure the current set of
abstractions, where we pass exactly one set of helper/callback
functions down through multiple functions is the best match for ext4.
It should be possible to significantly simplify the call stack by
reworking the GETFSMAP support.

On the other hand, the current, more complicated design might be
useful if at some point in the future, we were to add support for
reverse mapping for online fsck and/or if we add reflink support.
(See how Darrick implemented GETFSMAP for xfs.)

     	 	 	     	      	  - Ted