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Date:   Sun, 4 Jun 2023 04:27:51 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     fsverity@lists.linux.dev, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH] fsverity: don't use bio_first_page_all() in
 fsverity_verify_bio()
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References: <20230604022101.48342-1-ebiggers@kernel.org>
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On Sat, Jun 03, 2023 at 07:21:01PM -0700, Eric Biggers wrote:
> bio_first_page_all(bio)->mapping->host is not compatible with large
> folios, since the first page of the bio is not necessarily the head page
> of the folio, and therefore it might not have the mapping pointer set.

Yes, that's true.  It is going to depend on the filesystem, since these
two bios are equivalent:

(folio->page[0], offset=0x4000, len=0x300)
(folio->page[4], offset=0, len=0x300)

and we don't yet have a rule that filesystems must construct one or the
other.  We probably _should_, but that was pretty low down my list of
things to care about right now.

> Therefore, move the dereference of ->mapping->host into
> verify_data_blocks(), which works with a folio.

Seems reasonable.

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>

> @@ -320,7 +321,6 @@ EXPORT_SYMBOL_GPL(fsverity_verify_blocks);
>   */
>  void fsverity_verify_bio(struct bio *bio)
>  {
> -	struct inode *inode = bio_first_page_all(bio)->mapping->host;

An alternative fix could be

	struct folio *first = page_folio(bio_first_page_all(bio));
	struct inode *inode = first->mapping->host;

Or we could add a bio_first_folio_all() that wraps that for you.