linux-kernel - Re: [PATCH] kvm: Faults which trigger IO release the mmap

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Date:	Wed, 17 Sep 2014 20:08:42 +0300
From:	Gleb Natapov <gleb@...nel.org>
To:	Andres Lagar-Cavilla <andreslc@...gle.com>
Cc:	Radim Krčmář <rkrcmar@...hat.com>,
	Gleb Natapov <gleb@...hat.com>, Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.de>,
	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Jianyu Zhan <nasa4836@...il.com>,
	Paul Cassella <cassella@...y.com>,
	Hugh Dickins <hughd@...gle.com>,
	Peter Feiner <pfeiner@...gle.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] kvm: Faults which trigger IO release the mmap_sem

On Wed, Sep 17, 2014 at 10:00:32AM -0700, Andres Lagar-Cavilla wrote:
> On Wed, Sep 17, 2014 at 4:42 AM, Gleb Natapov <gleb@...nel.org> wrote:
> > On Wed, Sep 17, 2014 at 01:27:14PM +0200, Radim Krčmář wrote:
> >> 2014-09-17 13:26+0300, Gleb Natapov:
> >> > For async_pf_execute() you do not need to even retry. Next guest's page fault
> >> > will retry it for you.
> >>
> >> Wouldn't that be a waste of vmentries?
> > This is how it will work with or without this second gup. Page is not
> > mapped into a shadow page table on this path, it happens on a next fault.
> 
> The point is that the gup in the async pf completion from the work
> queue will not relinquish the mmap semaphore. And it most definitely
> should, given that we are likely looking at swap/filemap.
> 
I get this point and the patch looks good in general, but my point is
that when _retry() is called from async_pf_execute() second gup is not
needed. In the original code gup is called to do IO and nothing else.
In your patch this is accomplished by the first gup already, so you
can skip second gup if pagep == nullptr.

--
			Gleb.
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