Re: [PATCH] btrfs: prevent remounting to v1 space cache for subpage mount

[Josef Bacik <josef@toxicpanda.com>]

On Mon, May 16, 2022 at 02:26:53PM +0800, Qu Wenruo wrote:
> Upstream commit 9f73f1aef98b ("btrfs: force v2 space cache usage for
> subpage mount") forces subpage mount to use v2 cache, to avoid
> deprecated v1 cache which doesn't support subpage properly.
> 
> But there is a loophole that user can still remount to v1 cache.
> 
> The existing check will only give users a warning, but not really
> prevents the users to do the remount.
> 
> Although remounting to v1 will not cause any problems since the v1 cache
> will always be marked invalid when mounted with a different page size,
> it's still better to prevent v1 cache at all for subpage mounts.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>  fs/btrfs/super.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index b1fdc6a26c76..1617528a3367 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -1985,6 +1985,14 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
>  	if (ret)
>  		goto restore;
>  
> +	/* V1 cache is not supported for subpage mount. */
> +	if (fs_info->sectorsize < PAGE_SIZE &&
> +	    btrfs_test_opt(fs_info, SPACE_CACHE)) {
> +		btrfs_warn(fs_info,
> +	"v1 space cache is not supported for page size %lu with sectorsize %u",
> +			   PAGE_SIZE, fs_info->sectorsize);

Shouldn't we be doing ret = -EINVAL; here?  Thanks,

Josef