Re: [PATCH] iomap: don't override sis->bdev in xfs_iomap_swapfile_activate

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Sun, Mar 01, 2020 at 07:49:25AM -0700, Christoph Hellwig wrote:
> The swapon code itself sets sis->bdev up early, and performs various check
> on the block devices.  Changing it later in the fact thus will cause a
> mismatch of capabilities and must be avoided.

What kind of mismatch?  Are you talking about the bdi_cap_* and
blk_queue_nonrot() logic in swapon()?  I wonder how much of that could
be moved to after the ->swapfile_activate call.

> The practical implication
> of this change is that it forbids swapping to the RT subvolume, which might
> have had all kinds of issues anyway.

<shrug> I didn't find any the one time I tried it on a pair of
homogeneous devices. :)

> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/xfs/xfs_aops.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> index 58e937be24ce..f9929a952ef1 100644
> --- a/fs/xfs/xfs_aops.c
> +++ b/fs/xfs/xfs_aops.c
> @@ -637,7 +637,6 @@ xfs_iomap_swapfile_activate(
>  	struct file			*swap_file,
>  	sector_t			*span)
>  {
> -	sis->bdev = xfs_inode_buftarg(XFS_I(file_inode(swap_file)))->bt_bdev;

That said, btrfs copypasta'd this when they ported to iomap swapfile, so
that needs fixing too.

--D

>  	return iomap_swapfile_activate(sis, swap_file, span,
>  			&xfs_read_iomap_ops);
>  }
> -- 
> 2.24.1
>