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Date: Tue, 27 Jan 2026 07:00:39 +0100
From: Christoph Hellwig <hch@lst.de>
To: Eric Biggers <ebiggers@kernel.org>
Cc: "Darrick J. Wong" <djwong@kernel.org>, Christoph Hellwig <hch@lst.de>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Christian Brauner <brauner@kernel.org>, Jan Kara <jack@suse.cz>,
	David Sterba <dsterba@suse.com>, Theodore Ts'o <tytso@mit.edu>,
	Jaegeuk Kim <jaegeuk@kernel.org>, Chao Yu <chao@kernel.org>,
	Andrey Albershteyn <aalbersh@redhat.com>,
	Matthew Wilcox <willy@infradead.org>, linux-fsdevel@vger.kernel.org,
	linux-btrfs@vger.kernel.org, linux-ext4@vger.kernel.org,
	linux-f2fs-devel@lists.sourceforge.net, fsverity@lists.linux.dev
Subject: Re: [PATCH 07/16] fsverity: don't issue readahead for non-ENOENT
 errors from __filemap_get_folio
Message-ID: <20260127060039.GA25321@lst.de>
References: <20260126045212.1381843-1-hch@lst.de> <20260126045212.1381843-8-hch@lst.de> <20260126191102.GO5910@frogsfrogsfrogs> <20260126205301.GD30838@quark>
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In-Reply-To: <20260126205301.GD30838@quark>
User-Agent: Mutt/1.5.17 (2007-11-01)

On Mon, Jan 26, 2026 at 12:53:01PM -0800, Eric Biggers wrote:
> Then for the final version in generic_readahead_merkle_tree(), one
> option would be:
> 
> 	struct folio *folio;
> 
> 	folio = __filemap_get_folio(inode->i_mapping, index, FGP_ACCESSED, 0);
> 	if (folio == ERR_PTR(-ENOENT) ||
> 	    (!IS_ERR(folio) && !folio_test_uptodate(folio))) {
> 		DEFINE_READAHEAD(ractl, NULL, NULL, inode->i_mapping, index);
> 
> 		page_cache_ra_unbounded(&ractl, nr_pages, 0);
> 	}
> 	if (!IS_ERR(folio))
> 		folio_put(folio);
> 
> Or as a diff from this series:

I ended up doing the second version (which is what I intended to do
anyway, but messed up the brace placement) in this patch.  It then
automatically carries over to the readahead split.

> 
> -	if (PTR_ERR(folio) == -ENOENT ||
> -	    !(IS_ERR(folio) && !folio_test_uptodate(folio))) {
> +	if (folio == ERR_PTR(-ENOENT) ||
> +	    (!IS_ERR(folio) && !folio_test_uptodate(folio))) {
> 
> (Note that PTR_ERR() shouldn't be used before it's known that the
> pointer is an error pointer.)

That's new to me, and I can't find anything in the documentation or
implementation suggesting that.  Your example code above also does
this as does plenty of code in the kernel elsewhere.