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Date:   Tue, 27 Jul 2021 03:05:36 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     NeilBrown <neilb@suse.de>
Cc:     Goldwyn Rodrigues <rgoldwyn@suse.de>,
        linux-fsdevel@vger.kernel.org, linux-btrfs@vger.kernel.org,
        linux-fscrypt@vger.kernel.org, linux-nfs@vger.kernel.org
Subject: Re: [PATCH] fs: reduce pointers while using file_ra_state_init()
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References: <20210726164647.brx3l2ykwv3zz7vr@fiona>
 <162733718119.4153.5949006309014161476@noble.neil.brown.name>
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On Tue, Jul 27, 2021 at 08:06:21AM +1000, NeilBrown wrote:
> You seem to be assuming that inode->i_mapping->host is always 'inode'.
> That is not the case.

Weeeelllll ... technically, outside of the filesystems that are
changed here, the only assumption in common code that is made is that
inode_to_bdi(inode->i_mapping->host->i_mapping->host) ==
inode_to_bdi(inode)

Looking at inode_to_bdi, that just means that they have the same i_sb.
Which is ... not true for character raw devices?
        if (++raw_devices[minor].inuse == 1)
                file_inode(filp)->i_mapping =
                        bdev->bd_inode->i_mapping;
but then, who's using readahead on a character raw device?  They
force O_DIRECT.  But maybe this should pass inode->i_mapping->host
instead of inode.

> In particular, fs/coda/file.c contains
> 
> 	if (coda_inode->i_mapping == &coda_inode->i_data)
> 		coda_inode->i_mapping = host_inode->i_mapping;
> 
> So a "coda_inode" shares the mapping with a "host_inode".
> 
> This is why an inode has both i_data and i_mapping.
> 
> So I'm not really sure this patch is safe.  It might break codafs.
> 
> But it is more likely that codafs isn't used, doesn't work, should be
> removed, and i_data should be renamed to i_mapping.

I think there's also something unusual going on with either ocfs2
or gfs2.  But yes, I don't understand the rules for when I need to
go from inode->i_mapping->host.