linux-kernel - f_mapping->host vs. f_path.dentry->d

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Message-ID: <f2b55d220703141228q11c93c2i9b0b86d8953ab8b8@mail.gmail.com>
Date:	Wed, 14 Mar 2007 12:28:01 -0700
From:	"Michael K. Edwards" <medwards.linux@...il.com>
To:	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: f_mapping->host vs. f_path.dentry->d_inode

It appears that there are (at least) two ways to get the inode (if
any) associated with an open file: f_mapping->host (pagecache?) and
f_path.dentry->d_inode (dentry cache?).  generic_file_llseek uses
f_mapping->host; everything else in read_write.c uses
f_path.dentry->d_inode.  do_sendfile checks for a null inode on its
input fd but not on its output fd; nothing else in read_write.c checks
for null inode.

Under what circumstances can f_mapping->host and f_path.dentry->d_inode differ?

Under what circumstances can either or both of these pointers be null?

Under what circumstances is it preferable to retrieve the inode from
f_mapping vs. f_path.dentry, either because they differ or because one
or the other is more likely to be warm in cache?

Cheers,
- Michael
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