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Subject: Re: [PATCH 1/2] docs: mm/gup: pin_user_pages.rst: add a "case 5"
To:     Matthew Wilcox <willy@infradead.org>
CC:     Andrew Morton <akpm@linux-foundation.org>,
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References: <20200529234309.484480-1-jhubbard@nvidia.com>
 <20200529234309.484480-2-jhubbard@nvidia.com>
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From:   John Hubbard <jhubbard@nvidia.com>
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On 2020-06-12 12:24, Matthew Wilcox wrote:
> On Fri, May 29, 2020 at 04:43:08PM -0700, John Hubbard wrote:
>> +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,
>> +access 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()
>> +    access the data within the pages
>> +    set_page_dirty_lock()
>> +    unpin_user_pages()
>> +
>> +INCORRECT (uses FOLL_GET calls):
>> +    get_user_pages()
>> +    access the data within the pages
>> +    set_page_dirty_lock()
>> +    put_page()
> 
> Why does this case need to pin?  Why can't it just do ...
> 
> 	get_user_pages()
> 	lock_page(page);
> 	... modify the data ...
> 	set_page_dirty(page);
> 	unlock_page(page);
> 

Yes, it could do that. And that would also make a good additional "correct"
example. Especially for the case of just dealing with a single page,
lock_page() has the benefit of completely fixing the problem *today*,
without waiting for the pin_user_pages*() handling improvements to get
implemented.

And it's also another (probably better) way to fix the vhost.c problem, than
commit 690623e1b496 ("vhost: convert get_user_pages() --> pin_user_pages()").

I'm inclined to leave vhost.c alone for now, unless someone really prefers
it to be changed, but to update the Case 5 documentation with your point
above. Sound about right?


thanks,
-- 
John Hubbard
NVIDIA