linux-kernel - Re: [RFC] page fault retry with NOPAGE

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Message-Id: <20060919200533.2874ce36.akpm@osdl.org>
Date:	Tue, 19 Sep 2006 20:05:33 -0700
From:	Andrew Morton <akpm@...l.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Mike Waychison <mikew@...gle.com>, linux-mm@...ck.org,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...l.org>
Subject: Re: [RFC] page fault retry with NOPAGE_RETRY

On Wed, 20 Sep 2006 11:57:09 +1000
Benjamin Herrenschmidt <benh@...nel.crashing.org> wrote:

> > You forget that the point of this optimisation is to undo mmap_sem while
> > waiting on the disk IO.  Once we've done that we cannot go looking at ptes
> > or vmas: another thread could have munapped the whole lot or anything. 
> > (And we always need to be afraid of use_mm()..)
> 
> Wait wait .. .we don't need to have the mmap sem to -look- at a PTE.

The pte resides in a pagetable page.  Once we've dropped mmap_sem, that
pagetable page might not be there any more: munmap() might have freed it. 
We have to retake mmap_sem, do a find_vma() and a new pagetable walk.

There are some optimisations we could make to avoid all of that in the
common case, but this is the conceptual behaviour.
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