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

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Message-ID: <30ce6691-fd70-76a2-8b61-86d207c88713@nvidia.com>
Date:   Thu, 22 Oct 2020 12:58:14 -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 10/22/20 4:49 AM, Matthew Wilcox wrote:
> On Tue, Oct 20, 2020 at 01:25:59AM -0700, John Hubbard wrote:
>> Should copy_to_guest() use pin_user_pages_unlocked() instead of gup_unlocked?
>> We wrote a  "Case 5" in Documentation/core-api/pin_user_pages.rst, just for this
>> situation, I think:
>>
>>
>> CASE 5: Pinning in order to write to the data within the page
>> -------------------------------------------------------------
>> Even though neither DMA nor Direct IO is involved, just a simple case of "pin,
>> write to a page's data, unpin" can cause a problem. Case 5 may be considered a
>> superset of Case 1, plus Case 2, plus anything that invokes that pattern. In
>> other words, if the code is neither Case 1 nor Case 2, it may still require
>> FOLL_PIN, for patterns like this:
>>
>> Correct (uses FOLL_PIN calls):
>>      pin_user_pages()
>>      write to the data within the pages
>>      unpin_user_pages()
> 
> Case 5 is crap though.  That bug should have been fixed by getting
> the locking right.  ie:
> 
> 	get_user_pages_fast();
> 	lock_page();
> 	kmap();
> 	set_bit();
> 	kunmap();
> 	set_page_dirty()
> 	unlock_page();
> 
> I should have vetoed that patch at the time, but I was busy with other things.
> 

It does seem like lock_page() is better, for now at least, because it
forces the kind of synchronization with file system writeback that is
still yet to be implemented for pin_user_pages().

Long term though, Case 5 provides an alternative way to do this
pattern--without using lock_page(). Also, note that Case 5, *in
general*, need not be done page-at-a-time, unlike the lock_page()
approach. Therefore, Case 5 might potentially help at some call sites,
either for deadlock avoidance or performance improvements.

In other words, once the other half of the pin_user_pages() plan is
implemented, either of these approaches should work.

Or, are you thinking that there is never a situation in which Case 5 is
valid?


thanks,
-- 
John Hubbard
NVIDIA