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Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification
To:     Jason Gunthorpe <jgg@ziepe.ca>, Peter Xu <peterx@redhat.com>
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From:   John Hubbard <jhubbard@nvidia.com>
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On 9/15/20 4:22 PM, Jason Gunthorpe wrote:
> On Tue, Sep 15, 2020 at 05:33:30PM -0400, Peter Xu wrote:
> 
>>> RDMA doesn't ever use !WRITE
>>>
>>> I'm guessing #5 is the issue, just with a different ordering. If the
>>> #3 pin_user_pages() preceeds the #2 fork, don't we get to the same #5?
>>
>> Right, but only if without MADV_DONTFORK?
> 
> Yes, results are that MADV_DONTFORK resolves the issue for initial
> tests. I should know if a bigger test suite passes in a few days.
>   
>>>> If this is a problem, we may still need the fix patch (maybe not as urgent as
>>>> before at least).  But I'd like to double confirm, just in case I miss some
>>>> obvious facts above.
>>>
>>> I'm worred that the sudden need to have MAD_DONTFORK is going to be a
>>> turn into a huge regression. It already blew up our first level of
>>> synthetic test cases. I'm worried what we will see when the
>>> application suite is run in a few months :\
>>
>> For my own preference I'll consider changing kernel behavior if the impact is
>> still under control (the performance report of 30%+ boost is also attractive
>> after the simplify-cow patch).  The other way is to maintain the old reuse
>> logic forever, that'll be another kind of burden.  Seems no easy way on either
>> side...
> 
> It seems very strange that a physical page exclusively owned by a
> process can become copied if pin_user_pages() is active and the
> process did fork() at some point.
> 
> Could the new pin_user_pages() logic help here? eg the
> GUP_PIN_COUNTING_BIAS stuff?
> 
> Could the COW code consider a refcount of GUP_PIN_COUNTING_BIAS + 1 as
> being owned by the current mm and not needing COW? The DMA pin would
> be 'invisible' for COW purposes?


Please do be careful to use the API, rather than the implementation. The
FOLL_PIN refcounting system results in being able to get a "maybe
DMA-pinned", or a "definitely not DMA-pinned", via this API call:

     static inline bool page_maybe_dma_pinned(struct page *page)

...which does *not* always use GUP_PIN_COUNTING_BIAS to provide that
answer.

thanks,
-- 
John Hubbard
NVIDIA