linux-kernel - Re: [RFC] mm: gup: add helper page_try_gup

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Date:   Tue, 5 Nov 2019 09:56:51 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Jerome Glisse <jglisse@...hat.com>, Hillf Danton <hdanton@...a.com>
Cc:     John Hubbard <jhubbard@...dia.com>, linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jan Kara <jack@...e.cz>,
        Mel Gorman <mgorman@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Christoph Hellwig <hch@....de>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [RFC] mm: gup: add helper page_try_gup_pin(page)

On 04.11.19 20:03, Jerome Glisse wrote:
> On Mon, Nov 04, 2019 at 06:20:50PM +0800, Hillf Danton wrote:
>>
>> On Sun, 3 Nov 2019 22:09:03 -0800 John Hubbard wrote:
>>> On 11/3/19 8:34 PM, Hillf Danton wrote:
>>> ...
>>>>>
>>>>> Well, as long as we're counting bits, I've taken 21 bits (!) to track
>>>>> "gupers". :)  More accurately, I'm sharing 31 bits with get_page()...please
>>>>
>>>> Would you please specify the reasoning of tracking multiple gupers
>>>> for a dirty page? Do you mean that it is all fine for guper-A to add
>>>> changes to guper-B's data without warning and vice versa?
>>>
>>> It's generally OK to call get_user_pages() on a page more than once.
>>
>> Does this explain that it's generally OK to gup pin a page under
>> writeback and then start DMA to it behind the flusher's back without
>> warning?
> 
> It can happens today, is it ok ... well no but we live in an imperfect
> world. GUP have been abuse by few device driver over the years and those
> never checked what it meant to use it so now we are left with existing
> device driver that we can not break that do wrong thing.
> 
> I personaly think that we should use bounce page for writeback so that
> writeback can still happens if a page is GUPed. John's patchset is the
> first step to be able to identify GUPed page and maybe special case them.
> 
>>
>>> And even though we are seeing some work to reduce the number of places
>>> in the kernel that call get_user_pages(), there are still lots of call sites.
>>> That means lots of combinations and situations that could result in more
>>> than one gup call per page.
>>>
>>> Furthermore, there is no mechanism, convention, documentation, nor anything
>>> at all that attempts to enforce "for each page, get_user_pages() may only
>>> be called once."
>>
>> What sense is this making wrt the data corruption resulting specifically
>> from multiple gup references?
> 
> Multiple GUP references do not imply corruption. Only one or more devices
> writing to the page while writeback is happening is a cause of corruption.
> Multiple device writting in the same page concurrently is like multiple
> CPU thread doing the same. Either the application/device drivers are doing
> this rightfully on purpose or the application has a bug. Either way it is
> not our problem (note here i am talking about userspace portion of the
> device driver).
> 

If I'm not completely off, we can have multiple GUP references easily by 
using KVM+VFIO.

-- 

Thanks,

David / dhildenb