Re: Can reverse VM locks?

[Mark Hemment <markhe@veritas.com>]

On Mon, 30 Jul 2001, Rik van Riel wrote:

> OK, I've been looking at the lock order reversal too,
> though for different reasons ;)
>
> On Mon, 2 Jul 2001 markhe@veritas.com wrote:
> > On Mon, 2 Jul 2001, Rik van Riel wrote:
> > > On Mon, 2 Jul 2001 markhe@veritas.com wrote:
> > >
> > > >   Anyone know of any places where reversing the lock ordering would break?
>
> Yes, very much true.  Now what I wanted to ask about:
> do you already have a patch which does this or should
> I write a patch which does the lock order reversal ?

  I did do it, only took a couple of hours, but didn't show any measurable
improvement on a four-way box so I put it on the back-burner.  It is
probably lying around on an off-lined disk somewhere - I'll try to dig it
out tomorrow (time for pub/home in the UK), or re-code it.

  Three points to note;
	1) Looked like it might allow for easily coding of per page-cache
	   line spinlocks (if we want to go there).
	2) I suspected the pagemap_lru_lock was still under heavy
	   contention (the reversal wouldn't have helped it).
	3) In filemap.c, the pagecache_lock and pagemap_lru_lock are far
	   too "close" - need to be L1 cached aligned.

Mark

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