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

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Message-ID: <CAJu=L599fJABew7Oe6Bmp40VzMXY9wnE7P=c1xFaa+cU_KSoMw@mail.gmail.com>
Date:	Wed, 17 Sep 2014 10:13:45 -0700
From:	Andres Lagar-Cavilla <andreslc@...gle.com>
To:	Gleb Natapov <gleb@...nel.org>
Cc:	Radim Krčmář <rkrcmar@...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 10:08 AM, Gleb Natapov <gleb@...nel.org> wrote:
> 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.

I see. However, if this function were to be used elsewhere in the
future, then the "if pagep == NULL don't retry" semantics may not
match the new caller's intention. Would you prefer an explicit flag?

Andres

>
> --
>                         Gleb.



-- 
Andres Lagar-Cavilla | Google Kernel Team | andreslc@...gle.com
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