Re: dentry bloat.

From: Dipankar Sarma
Date: Sat May 08 2004 - 16:04:20 EST

On Sat, May 08, 2004 at 12:01:48PM -0700, Andrew Morton wrote:
> Linus Torvalds <torvalds@xxxxxxxx> wrote:
> > Also, in your previous patch (which I'm not as convinced might be wrong),
> > the d_qstr pointer removal makes me worry:
> >
> > - struct qstr * d_qstr; /* quick str ptr used in lockless lookup and concurrent d_move */
> >
> > I thought the point of d_qstr was that when we do the lockless lookup,
> > we're guaranteed to always see "stable storage" in the sense that when we
> > follow the d_qstr, we will always get a "char *" + "len" that match, and
> > we could never see a partial update (ie len points to the old one, and
> > "char *" points to the new one).
> It looks that way.

Yes, that is exactly why d_qstr was introduced. The "len" and the
storage for the name is then a single update through d_qstr.

> > In particular, think about the "d_compare(parent, qstr, name)" /
> > "memcmp(qstr->name, str, len)" part - what if "len" doesn't match str,
> > because a concurrent d_move() is updating them, and maybe we will compare
> > past the end of kernel mapped memory or something?
> >
> > (In other words, the "move_count" check should protect us from returning a
> > wrong dentry, but I'd worry that we'd do something that could cause
> > serious problems before we even get to the "move_count" check).
> >
> > Hmm?

Yes, that is indeed why we had to have d_qstr.

> I think we can simply take ->d_lock a bit earlier in __d_lookup. That will
> serialise against d_move(), fixing the problem which you mention, and also
> makes d_movecount go away.

Repeating some of the tests under ->d_lock is worth looking at, but
we have to be carefull about performance. ISTR, there was another
issue related to calling ->d_compare() under ->d_lock. I will dig
a little bit on this, or Maneesh may remember.

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