linux-ext4 - Re: [RFC v6 04/10] iomap: Add iomap_get

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Date:   Tue, 10 Jan 2023 13:34:16 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Andreas Gruenbacher <agruenba@...hat.com>,
        Dave Chinner <dchinner@...hat.com>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, cluster-devel@...hat.com,
        Christoph Hellwig <hch@....de>
Subject: Re: [RFC v6 04/10] iomap: Add iomap_get_folio helper

On Tue, Jan 10, 2023 at 12:46:45AM -0800, Christoph Hellwig wrote:
> On Mon, Jan 09, 2023 at 01:46:42PM +0100, Andreas Gruenbacher wrote:
> > We can handle that by adding a new IOMAP_NOCREATE iterator flag and
> > checking for that in iomap_get_folio().  Your patch then turns into
> > the below.
> 
> Exactly.  And as I already pointed out in reply to Dave's original
> patch what we really should be doing is returning an ERR_PTR from
> __filemap_get_folio instead of reverse-engineering the expected
> error code.

Ouch, we have a nasty problem.

If somebody passes FGP_ENTRY, we can return a shadow entry.  And the
encodings for shadow entries overlap with the encodings for ERR_PTR,
meaning that some shadow entries will look like errors.  The way I
solved this in the XArray code is by shifting the error values by
two bits and encoding errors as XA_ERROR(-ENOMEM) (for example).

I don't _object_ to introducing XA_ERROR() / xa_err() into the VFS,
but so far we haven't, and I'd like to make that decision intentionally.