linux-kernel - Re: [GIT PULL] KVM updates for the 2.6.38 merge window

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Message-ID: <AANLkTimi9ZfO+8rGuBSKdSiXSZmQfvs8eceYRgdpdO6s@mail.gmail.com>
Date:	Thu, 13 Jan 2011 07:43:55 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Rik van Riel <riel@...hat.com>, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [GIT PULL] KVM updates for the 2.6.38 merge window

On Thu, Jan 13, 2011 at 4:53 AM, Gleb Natapov <gleb@...hat.com> wrote:
>
> I implemented get_user_pages_nowait() on top of your patch. In my testing
> it works as expected when used inside KVM. Does this looks OK to you?

It looks reasonable, although I suspect the subtle behavior wrt the
mmap_sem means that you should not expose the magic bare
FAULT_FLAG_ALLOW_RETRY flag to the __get_user_pages() thing. It's just
too easy to introduce bugs, methinks.

So I'd suggest

 - drop FOLL_RETRY

 -  make FOLL_NOWAIT set both (FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_NOWAIT)

and that way the get_user_pages() thing will never release the
mmap_sem, and you never have any subtle locking issues for that
particular interface.

But some other VM person should look at it too.

                                 Linus
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