Re: dentry bloat.

From: viro
Date: Sat May 08 2004 - 05:29:21 EST

On Sat, May 08, 2004 at 03:11:59AM -0700, Andrew Morton wrote:
> Andrew Morton <akpm@xxxxxxxx> wrote:
> >
> > Here be another patch.
> Can't help myself!
> - d_vfs_flags can be removed - just use d_flags. It looks like someone
> added d_vfs_flags with the intent of doing bitops on it, but all
> modifications are under dcache_lock. 4 bytes saved.

Bzzert. ->d_vfs_flags modifications are under dcache_lock; ->d_flags ones
are *not* - they are up to whatever filesystem code happens to use them.

> - Pack things so that dentry.d_name.len and dentry.d_flags occupy the same
> word. 4 bytes saved.

d_name.len is accessed on very hot paths. In particular, we are looking
at it while traversing hash chains and we do that without dcache_lock
(see callers of ->d_compare()).

If we are going to hold dcache_lock in __d_lookup(), we can get rid of
a _lot_ more than 2 bytes. 32 bytes that came from RCU are there pretty
much for a single reason - to avoid dcache_lock on that path.
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