f_mapping->host vs. f_path.dentry->d_inode

[Michael K. Edwards]

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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