linux-kernel - Re: [RFCv2 08/16] KVM: Use GUP instead of copy_from/to

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Date:   Mon, 26 Oct 2020 13:52:07 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        "Peter Zijlstra" <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Sean Christopherson" <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        David Rientjes <rientjes@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Will Drewry <wad@...omium.org>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "Kleen, Andi" <andi.kleen@...el.com>,
        "Liran Alon" <liran.alon@...cle.com>,
        Mike Rapoport <rppt@...nel.org>, <x86@...nel.org>,
        <kvm@...r.kernel.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFCv2 08/16] KVM: Use GUP instead of copy_from/to_user() to
 access guest memory

On 10/26/20 6:28 AM, Matthew Wilcox wrote:
> On Sun, Oct 25, 2020 at 09:44:07PM -0700, John Hubbard wrote:
>> On 10/25/20 9:21 PM, Matthew Wilcox wrote:
>>> I don't think the page pinning approach is ever valid.  For file
>>
>> Could you qualify that? Surely you don't mean that the entire pin_user_pages
>> story is a waste of time--I would have expected you to make more noise
>> earlier if you thought that, yes?
> 
> I do think page pinning is the wrong approach for everything.  I did say

Not *everything*, just "pinning for DMA", right? Because I don't recall
any viable solutions for Direct IO that avoided gup/pup!

Also, back to Case 5: I *could* create a small patchset to change over
the very few Case 5 call sites to use "gup, lock_page(), write to
page...etc", instead of pup. And also, update pin_user_pages.rst to
recommend that approach in similar situations. After all, it's not
really a long-term DMA pin, which is really what pin_user_pages*() is
intended for.

Would that be something you'd like to see happen? It's certainly easy
enough to fix that up. And your retroactive NAK is sufficient motivation
to do so.

> so at the time, and I continue to say so when the opportunity presents
> itself.  But shouting about it constantly only annoys people, so I don't
> generally bother.  I have other things to work on, and they're productive,
> so I don't need to spend my time arguing.

Sure. As a practical matter, I've assumed that page pinning is not going
to go away any time soon, so I want it to work properly while it's here.
But if there is a viable way to eventually replace dma-pinning with
something better, then let's keep thinking about it. I'm glad to help in
that area.

thanks,
-- 
John Hubbard
NVIDIA