Re: [PATCH v1 07/23] erofs: add nodev mode

[Gao Xiang <hsiangkao@linux.alibaba.com>]

On Mon, Dec 27, 2021 at 08:54:28PM +0800, Jeffle Xu wrote:
> Until then erofs is exactly blockdev based filesystem. In other using
> scenarios (e.g. container image), erofs needs to run upon files.
> 
> This patch introduces a new nodev mode, in which erofs could be mounted
> from a bootstrap blob file containing the complete erofs image.
> 
> The following patch will introduce a new mount option "uuid", by which
> users could specify the bootstrap blob file.
> 
> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>

I think the order of some patches in this patchset can be improved.

Take this patch as an example. This patch introduces a new mount
option called "uuid", so the kernel will just accept it (which
generates a user-visible impact) after this patch but it doesn't
actually work.

Therefore, we actually have three different behaviors here:
 - kernel doesn't support "uuid" mount option completely;
 - kernel support "uuid" but it doesn't work;
 - kernel support "uuid" correctly (maybe after some random patch);

Actually that is bad for bisecting since there are some commits
having temporary behaviors. And we don't know which commit
actually fully implements this "uuid" mount option.

So personally I think the proper order is just like the bottom-up
approach, and make sure each patch can be tested / bisected
independently.

> ---
>  fs/erofs/data.c     | 13 ++++++++---
>  fs/erofs/internal.h |  1 +
>  fs/erofs/super.c    | 56 +++++++++++++++++++++++++++++++++------------
>  3 files changed, 53 insertions(+), 17 deletions(-)
> 
> diff --git a/fs/erofs/data.c b/fs/erofs/data.c
> index 477aaff0c832..61fa431d0713 100644
> --- a/fs/erofs/data.c
> +++ b/fs/erofs/data.c
> @@ -11,11 +11,18 @@
>  
>  struct page *erofs_get_meta_page(struct super_block *sb, erofs_blk_t blkaddr)
>  {
> -	struct address_space *const mapping = sb->s_bdev->bd_inode->i_mapping;
> +	struct address_space *mapping;
>  	struct page *page;
>  
> -	page = read_cache_page_gfp(mapping, blkaddr,
> -				   mapping_gfp_constraint(mapping, ~__GFP_FS));

Apart from the recommendation above, if my understanding is
correct, I think after we implement fscache_aops, 
read_cache_page_gfp() can work with proper fscache mapping.

So no need to implement something like erofs_readpage_from_fscache()
later (at least for the case here.)

Thanks,
Gao Xiang

> +	if (sb->s_bdev) {
> +		mapping = sb->s_bdev->bd_inode->i_mapping;
> +		page = read_cache_page_gfp(mapping, blkaddr,
> +				mapping_gfp_constraint(mapping, ~__GFP_FS));
> +	} else {
> +		/* TODO: data path in nodev mode */
> +		page = ERR_PTR(-EINVAL);
> +	}
> +