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Date: Mon, 26 Jan 2026 22:28:49 -0800
From: "Darrick J. Wong" <djwong@kernel.org>
To: Eric Biggers <ebiggers@kernel.org>
Cc: 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: <20260127062849.GX5966@frogsfrogsfrogs>
References: <20260126045212.1381843-1-hch@lst.de>
 <20260126045212.1381843-8-hch@lst.de>
 <20260126191102.GO5910@frogsfrogsfrogs>
 <20260126205301.GD30838@quark>
 <20260127060039.GA25321@lst.de>
 <20260127062055.GA90735@sol>
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In-Reply-To: <20260127062055.GA90735@sol>

On Mon, Jan 26, 2026 at 10:20:55PM -0800, Eric Biggers wrote:
> On Tue, Jan 27, 2026 at 07:00:39AM +0100, Christoph Hellwig wrote:
> > > -	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.
> 
> Not sure why this is controversial.  The documentation for PTR_ERR() is
> clear that it's for error pointers:
> 
> /**
>  * PTR_ERR - Extract the error code from an error pointer.
>  * @ptr: An error pointer.
>  * Return: The error code within @ptr.
>  */
> static inline long __must_check PTR_ERR(__force const void *ptr)
> {
>         return (long) ptr;
> }
> 
> Yes, it's really just a cast, and 'PTR_ERR(folio) == -ENOENT' actually
> still works when folio isn't necessarily an error pointer.  But normally
> it would be written as a pointer comparison as I suggested.

How does one know that a pointer is an error pointer?  Oughtn't there be
some kind of obvious marker, or is IS_ERR the only tool we've got?

--D

> - Eric