linux-kernel - Re: GRU driver feedback

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Date:	Thu, 31 Jul 2008 17:14:04 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Robin Holt <holt@....com>
Cc:	"Torvalds, Linus" <torvalds@...ux-foundation.org>,
	Jack Steiner <steiner@....com>, Nick Piggin <npiggin@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: GRU driver feedback

On Wednesday 30 July 2008 15:50, Nick Piggin wrote:
> On Wednesday 30 July 2008 04:53, Robin Holt wrote:

> > In the case where unmap_region is clearing page tables, the caller to
> > unmap_region is expected to be holding the mmap_sem writably.  Jacks
> > fault handler will immediately return when it fails on the
> > down_read_trylock().
>
> No, you are right of course. I had in my mind the problems faced by
> lockless get_user_pages, in which case I was worried about the page table
> existence, but missed the fact that you're holding mmap_sem to provide
> existence (which it would, as you note, although one day we may want to
> reclaim page tables or something that doesn't take mmap_sem, so a big
> comment would be nice here).

The other thing is... then GRU should get rid of the local_irq_disable
in the atomic pte lookup. By definition it is worthless if we can be
operating on an mm that is not running on current (and if I understand
correctly, sn2 can avoid sending tlb flush IPIs completely sometimes?)
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