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Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions
To:     Jan Kara <jack@suse.cz>
CC:     Jerome Glisse <jglisse@redhat.com>,
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From:   John Hubbard <jhubbard@nvidia.com>
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On 1/29/19 2:12 AM, Jan Kara wrote:
> On Mon 28-01-19 22:41:41, John Hubbard wrote:
[...]
>> Here is the case I'm wondering about:
>>
>> thread A                             thread B
>> --------                             --------
>>                                      gup_fast
>> page_mkclean
>>     is page gup-pinned?(no)
>>                                          page_cache_get_speculative
>>                                              (gup-pins the page here)
>>                                          check pte_val unchanged (yes)
>>        set_pte_at()
>>
>> ...and now thread A has created a read-only PTE, after gup_fast walked
>> the page tables and found a writeable entry. And so far, thread A has
>> not seen that the page is pinned.
>>
>> What am I missing here? The above seems like a problem even before we
>> change anything.
> 
> Your implementation of page_mkclean() is wrong :) It needs to first call
> set_pte_at() and only after that ask "is page gup pinned?". In fact,
> page_mkclean() probably has no bussiness in checking for page pins
> whatsoever. It is clear_page_dirty_for_io() that cares, so that should
> check for page pins after page_mkclean() has returned.
> 

Perfect, that was the missing piece for me: page_mkclean() internally doesn't
need the consistent view, just the caller does. The whole situation with
two distinct lock-free algorithms going on here actually seems clear at last. :)

Thanks (also to Jerome) for explaining this!

thanks,
-- 
John Hubbard
NVIDIA