Re: [PATCH] zonefs: move super block reading from page to folio

[Matthew Wilcox <willy@infradead.org>]

On Tue, May 14, 2024 at 05:22:08PM +0200, Johannes Thumshirn wrote:
> From: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> 
> Move reading of the on-disk superblock from page to kmalloc()ed memory.

No, this is wrong.

> +	super = kzalloc(ZONEFS_SUPER_SIZE, GFP_KERNEL);
> +	if (!super)
>  		return -ENOMEM;
>  
> +	folio = virt_to_folio(super);

This will stop working at some point.  It'll return NULL once we get
to the memdesc future (because the memdesc will be a slab, not a folio).

>  	bio_init(&bio, sb->s_bdev, &bio_vec, 1, REQ_OP_READ);
>  	bio.bi_iter.bi_sector = 0;
> -	__bio_add_page(&bio, page, PAGE_SIZE, 0);
> +	bio_add_folio_nofail(&bio, folio, ZONEFS_SUPER_SIZE,
> +			     offset_in_folio(folio, super));

It also doesn't solve the problem of trying to read 4KiB from a device
with 16KiB sectors.  We'll have to fail the bio because there isn't
enough memory in the bio to store one block.

I think the right way to handle this is to call read_mapping_folio().
That will allocate a folio in the page cache for you (obeying the
minimum folio size).  Then you can examine the contents.  It should
actually remove code from zonefs.  Don't forget to call folio_put()
when you're done with it (either at unmount or at the end of mount if
you copy what you need elsewhere).