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From: John Hubbard <jhubbard@nvidia.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	"Peter Zijlstra" <peterz@infradead.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	"Sean Christopherson" <sean.j.christopherson@intel.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	David Rientjes <rientjes@google.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Kees Cook <keescook@chromium.org>, Will Drewry <wad@chromium.org>,
	"Edgecombe, Rick P" <rick.p.edgecombe@intel.com>,
	"Kleen, Andi" <andi.kleen@intel.com>,
	"Liran Alon" <liran.alon@oracle.com>,
	Mike Rapoport <rppt@kernel.org>, <x86@kernel.org>,
	<kvm@vger.kernel.org>, <linux-mm@kvack.org>,
	<linux-kernel@vger.kernel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [RFCv2 08/16] KVM: Use GUP instead of copy_from/to_user() to access guest memory
Date: Sun, 25 Oct 2020 21:44:07 -0700	[thread overview]
Message-ID: <ee308d1d-8762-6bcf-193e-85fea29743c3@nvidia.com> (raw)
In-Reply-To: <20201026042158.GN20115@casper.infradead.org>

On 10/25/20 9:21 PM, Matthew Wilcox wrote:
> On Thu, Oct 22, 2020 at 12:58:14PM -0700, John Hubbard wrote:
>> 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?
> 
> 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?

> mappings, the infiniband registration should take a lease on the inode,
> blocking truncation.  DAX shouldn't be using struct pages at all, so
> there shouldn't be anything there to pin.
> 
> 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? :)



thanks,
-- 
John Hubbard
NVIDIA


  reply	other threads:[~2020-10-26  4:44 UTC|newest]

Thread overview: 57+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-20  6:18 [RFCv2 00/16] KVM protected memory extension Kirill A. Shutemov
2020-10-20  6:18 ` [RFCv2 01/16] x86/mm: Move force_dma_unencrypted() to common code Kirill A. Shutemov
2020-10-20  6:18 ` [RFCv2 02/16] x86/kvm: Introduce KVM memory protection feature Kirill A. Shutemov
2020-10-20  6:18 ` [RFCv2 03/16] x86/kvm: Make DMA pages shared Kirill A. Shutemov
2020-10-20  6:18 ` [RFCv2 04/16] x86/kvm: Use bounce buffers for KVM memory protection Kirill A. Shutemov
2020-10-20  6:18 ` [RFCv2 05/16] x86/kvm: Make VirtIO use DMA API in KVM guest Kirill A. Shutemov
2020-10-20  8:06   ` Christoph Hellwig
2020-10-20 12:47     ` Kirill A. Shutemov
2020-10-22  3:31   ` Halil Pasic
2020-10-20  6:18 ` [RFCv2 06/16] x86/kvmclock: Share hvclock memory with the host Kirill A. Shutemov
2020-10-20  6:18 ` [RFCv2 07/16] x86/realmode: Share trampoline area if KVM memory protection enabled Kirill A. Shutemov
2020-10-20  6:18 ` [RFCv2 08/16] KVM: Use GUP instead of copy_from/to_user() to access guest memory Kirill A. Shutemov
2020-10-20  8:25   ` John Hubbard
2020-10-20 12:51     ` Kirill A. Shutemov
2020-10-22 11:49     ` Matthew Wilcox
2020-10-22 19:58       ` John Hubbard
2020-10-26  4:21         ` Matthew Wilcox
2020-10-26  4:44           ` John Hubbard [this message]
2020-10-26 13:28             ` Matthew Wilcox
2020-10-26 14:16               ` Jason Gunthorpe
2020-10-26 20:52               ` John Hubbard
2020-10-20 17:29   ` Ira Weiny
2020-10-22 11:37     ` Kirill A. Shutemov
2020-10-20  6:18 ` [RFCv2 09/16] KVM: mm: Introduce VM_KVM_PROTECTED Kirill A. Shutemov
2020-10-21 18:47   ` Edgecombe, Rick P
2020-10-22 12:01     ` Kirill A. Shutemov
2020-10-20  6:18 ` [RFCv2 10/16] KVM: x86: Use GUP for page walk instead of __get_user() Kirill A. Shutemov
2020-10-20  6:18 ` [RFCv2 11/16] KVM: Protected memory extension Kirill A. Shutemov
2020-10-20  7:17   ` Peter Zijlstra
2020-10-20 12:55     ` Kirill A. Shutemov
2020-10-20  6:18 ` [RFCv2 12/16] KVM: x86: Enabled protected " Kirill A. Shutemov
2020-10-20  6:18 ` [RFCv2 13/16] KVM: Rework copy_to/from_guest() to avoid direct mapping Kirill A. Shutemov
2020-10-20  6:18 ` [RFCv2 14/16] KVM: Handle protected memory in __kvm_map_gfn()/__kvm_unmap_gfn() Kirill A. Shutemov
2020-10-21 18:50   ` Edgecombe, Rick P
2020-10-22 12:06     ` Kirill A. Shutemov
2020-10-22 16:59       ` Edgecombe, Rick P
2020-10-23 10:36         ` Kirill A. Shutemov
2020-10-22  3:26   ` Halil Pasic
2020-10-22 12:07     ` Kirill A. Shutemov
2020-10-20  6:18 ` [RFCv2 15/16] KVM: Unmap protected pages from direct mapping Kirill A. Shutemov
2020-10-20  7:12   ` Peter Zijlstra
2020-10-20 12:18   ` David Hildenbrand
2020-10-20 13:20     ` David Hildenbrand
2020-10-21  1:20       ` Edgecombe, Rick P
2020-10-26 19:55     ` Tom Lendacky
2020-10-21 18:49   ` Edgecombe, Rick P
2020-10-23 12:37   ` Mike Rapoport
2020-10-23 16:32     ` Sean Christopherson
2020-10-20  6:18 ` [RFCv2 16/16] mm: Do not use zero page for VM_KVM_PROTECTED VMAs Kirill A. Shutemov
2020-10-20  7:46 ` [RFCv2 00/16] KVM protected memory extension Vitaly Kuznetsov
2020-10-20 13:49   ` Kirill A. Shutemov
2020-10-21 14:46     ` Vitaly Kuznetsov
2020-10-23 11:35       ` Kirill A. Shutemov
2020-10-23 12:01         ` Vitaly Kuznetsov
2020-10-21 18:20 ` Andy Lutomirski
2020-10-26 15:29   ` Kirill A. Shutemov
2020-10-26 23:58     ` Andy Lutomirski

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