Re: [PATCH][RFC] %pd - for printing dentry name

[Al Viro <viro@ZenIV.linux.org.uk>]

On Thu, Feb 04, 2010 at 09:13:18AM -0800, Linus Torvalds wrote:
> 
> 
> On Thu, 4 Feb 2010, Paul E. McKenney wrote:
> > 
> > Ah, good point on the hash size.  And given that DNAME_INLINE_LEN_MIN
> > is 40 characters on 32-bit systems and 32 characters on 64-bit systems,
> > I agree that while a four-character increase might be nice, it cannot be
> > said to be an emergency.
> 
> Well, what we _could_ do is to make the 'length' field be part of the name 
> itself, and just keep the hash separate. The hash (and parenthood) is what 
> we check most in the hot inner loop, and don't want to have any extra 
> indirection (or cache misses) for. The name length we check only later, 
> after we've done all other checks (and after we've gotten the spinlock, 
> which is the big thing).
> 
> So qstr->len is _not_ performance critical in the same way that qstr->hash 
> is.

We could also try to put the hash chain in that sucker, copy d_parent in
there *and* put a pointer back to struct dentry in it.  Then the walk
itself would go through those and we'd actually looked at the dentry
only once - in the end of it.  Normally that thing would be just embedded
into dentry, with ability to allocate separately.

That might deal with lockless lookups if we did it right, but delayed
copying back into dentry and freeing of out-of-line copy (after d_move())
would still cause all sorts of fun.

The thing is, we have places where ->d_name.name uses rely on "I hold
i_mutex on parent, so this thing won't change or go away under me" and
that's actually the majority of code using ->d_name.  All directory
operations.

How about doing that delayed work just before dropping i_mutex on parent?
There we definitely can sleep, etc., so if we have d_move mark dentry as
"got out-of-line hash chain+name+hash+len+d_parent_copy, want to collapse
it back into dentry" and do d_collapse_that_stuff(dentry) before the
matching drop of i_mutex...

It would be one hell of a patch size, probably, but it seems that the rest
of problems wouldn't be there...  All such out-of-line structs would be
freed via RCU and never modified.  And inline ones would be modified only
when
	a) everyone who looks at hash chains already sees out-of-line one
	b) i_mutex on parent is still held
They'd get out-of-line one copied into them, replace it in hash chains
and schedule freeing of out-of-line sucker.

The reason why I'm talking about copy of d_parent and not just taking the
field over there: we avoid messing with dentry refcounting, etc. that way,
assuming that this copy is never dereferenced (used only for comparisons
during dcache lookups) and doesn't contribute to d_count.  Freeing dentries
themselves would be also RCU-delayed, of course.

Comments?