linux-kernel - coda's use of file->f_mapping and inode->i

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Message-ID: <1501686331.4654.6.camel@redhat.com>
Date:   Wed, 02 Aug 2017 11:05:31 -0400
From:   Jeff Layton <jlayton@...hat.com>
To:     Jan Harkes <jaharkes@...cmu.edu>, coda@...cmu.edu
Cc:     codalist@...a.cs.cmu.edu,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: coda's use of file->f_mapping and inode->i_mapping

I've been slowly crawling through filesystems to convert them to
errseq_t based error handling for ->fsync operations. I started looking
at coda, but it does some strange things with the f_mapping that I don't
quite understand.

When a file is opened on coda, we call down to userland daemon, which
opens the file and passes the fd back to the kernel. The kernel then
converts that to a struct file pointer and stores that in the
coda_file_info->cfi_container. So far, so good...

The weird bit is that in coda_file_mmap, we then do this:

        coda_file->f_mapping = host_file->f_mapping;
        if (coda_inode->i_mapping == &coda_inode->i_data)
                coda_inode->i_mapping = host_inode->i_mapping;

What is the significance of mmap on coda files? If you want to monkey
around with the i_mapping and f_mapping, wouldn't it make more sense to
do so at open() time?

-- 
Jeff Layton <jlayton@...hat.com>