Re: [RFC PATCH 19/19] tarfs: introduce tar fs

[Matthew Wilcox <willy@infradead.org>]

On Thu, Jan 25, 2024 at 08:05:25AM +1100, Dave Chinner wrote:
> On Wed, Jan 24, 2024 at 03:26:03PM -0300, Wedson Almeida Filho wrote:
> > So what is the recommended way? Which file systems are using it (so I
> > can do something similar)?
> 
> e.g. btrfs_read_dev_one_super(). Essentially, if your superblock is
> at block zero in the block device:
> 
> 	struct address_space *mapping = bdev->bd_inode->i_mapping;
> 
> 	......
> 
> 	page = read_cache_page_gfp(mapping, 0, GFP_NOFS);
>         if (IS_ERR(page))
>                 return ERR_CAST(page);
> 
>         super = page_address(page);
> 
> And now you have a pointer to your in memory buffer containing the
> on-disk superblock. If the sueprblock is not at block zero, then
> replace the '0' passed to read_cache_page_gfp() with whatever page
> cache index the superblock can be found at....

Just to modify this slightly ...

	folio = read_mapping_folio(mapping, pos / PAGE_SIZE);
	if (IS_ERR(folio))
		return ERR_CAST(folio);
	super = folio_address(folio) + offset_in_folio(folio, pos);

... and then in your shutdown path, you'll need to call folio_put().
Maybe that's easiest done in Rust by "leaking" the folio into the
in-memory super block so it doesn't get dropped at the end of the
function?

I don't think you need the GFP_NOFS.  We don't have a superblock yet, so
we can't call back into the filesystem.