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

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Date:   Mon, 26 Oct 2020 13:28:30 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     John Hubbard <jhubbard@...dia.com>
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 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
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.

> > It's been five years since DAX was merged, and page pinning still
> > doesn't work.  How much longer before the people who are pushing it
> > realise that it's fundamentally flawed?
> 
> Is this a separate rant about *only* DAX, or is general RDMA in your sights
> too? :)

This is a case where it's not RDMA's _fault_ that there's no good API
for it to do what it needs to do.  There's a lot of work needed to wean
Linux device drivers off their assumption that there's a struct page
for every byte of memory.