Re: [PATCH 03/21] fs/bdev: Add atomic write support info to statx

[John Garry]

On 03/10/2023 16:46, Darrick J. Wong wrote:
> >      stat->result_mask |= STATX_WRITE_ATOMIC;
> The result_mask (which becomes the statx stx_mask) needs to have
> STATX_WRITE_ATOMIC set any time a filesystem responds to
> STATX_WRITE_ATOMIC being set in the request_mask, even if the response
> is "not supported".
>
> The attributes_mask also needs to have STATX_ATTR_WRITE_ATOMIC set if
> the filesystem+file can support the flag, even if it's not currently set
> for that file.  This should get turned into a generic vfs helper for the
> next fs that wants to support atomic write units:
>
> static void generic_fill_statx_atomic_writes(struct kstat *stat,
> 		struct block_device *bdev)
> {
> 	u64 min_bytes;
>
> 	/* Confirm that the fs driver knows about this statx request */
> 	stat->result_mask |= STATX_WRITE_ATOMIC;
>
> 	/* Confirm that the file attribute is known to the fs. */
> 	stat->attributes_mask |= STATX_ATTR_WRITE_ATOMIC;
>
> 	/* Fill out the rest of the atomic write fields if supported */
> 	min_bytes = queue_atomic_write_unit_min_bytes(bdev->bd_queue);
> 	if (min_bytes == 0)
> 		return;
>
> 	stat->atomic_write_unit_min = min_bytes;
> 	stat->atomic_write_unit_max =
> 			queue_atomic_write_unit_max_bytes(bdev->bd_queue);
>
> 	/* Atomic writes actually supported on this file. */
> 	stat->attributes |= STATX_ATTR_WRITE_ATOMIC;
> }
>
> and then:
>
> 	if (request_mask & STATX_WRITE_ATOMIC)
> 		generic_fill_statx_atomic_writes(stat, bdev);

That looks sensible, but, if used by an FS, we would still need a method 
to include extra FS restrictions, like extent alignment as in 15/21.

Thanks,
John