On Fri, Jun 18, 2010 at 03:04:08AM +0900, J. R. Okajima wrote:
What I'm hearing again and again is that d_ino is useful to improve
performance. As Andreas put it to me, if d_ino is the same, the
referenced file may or may not be the same, but if it's different, the
files are definitely different. Only in well-controlled environments
known not to have submounts or bind mounts do people trust d_ino to be
from the same file system as the other entries in a directory.
I only submitted this patch half-seriously - mainly I wanted to find
out how people are using d_ino, and therefore what I need to do for
fallthru directory entries in union mounts.
In order to get the correct inode number for a directory entry
referring to a lower layer file or directory, we have to do a
->lookup() from the fs-specific readdir code (or else require that
fallthrus store an arbitrarily sized integer - which seriously
restricts the implementation). Now, doing a ->lookup() to get d_ino
makes no sense if we are using d_ino as a way to avoid the cost
stat(), which is mainly the ->lookup(). And you definitely can't use
d_ino by itself in a union mount.
I'm inclined to save the trouble and just return 1 in d_ino for
fallthru directory entries, especially now that I've tested it
system-wide and had no obvious problems.
-VAL
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