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Subject: Re: [PATCH v4 2/3] mm: introduce put_user_page*(), placeholder
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To:     Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.cz>
CC:     <john.hubbard@gmail.com>, Matthew Wilcox <willy@infradead.org>,
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From:   John Hubbard <jhubbard@nvidia.com>
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On 10/9/18 4:20 PM, Andrew Morton wrote:
> On Tue, 9 Oct 2018 10:30:25 +0200 Jan Kara <jack@suse.cz> wrote:
> 
>>> Also, maintainability.  What happens if someone now uses put_page() by
>>> mistake?  Kernel fails in some mysterious fashion?  How can we prevent
>>> this from occurring as code evolves?  Is there a cheap way of detecting
>>> this bug at runtime?
>>
>> The same will happen as with any other reference counting bug - the special
>> user reference will leak. It will be pretty hard to debug I agree. I was
>> thinking about whether we could provide some type safety against such bugs
>> such as get_user_pages() not returning struct page pointers but rather some
>> other special type but it would result in a big amount of additional churn
>> as we'd have to propagate this different type e.g. through the IO path so
>> that IO completion routines could properly call put_user_pages(). So I'm
>> not sure it's really worth it.
> 
> I'm not really understanding.  Patch 3/3 changes just one infiniband
> driver to use put_user_page().  But the changelogs here imply (to me)
> that every user of get_user_pages() needs to be converted to
> s/put_page/put_user_page/.
> 
> Methinks a bit more explanation is needed in these changelogs?
> 

OK, yes, it does sound like the explanation is falling short. I'll work on something 
clearer. Did the proposed steps in the changelogs, such as:
  
[2] https://lkml.kernel.org/r/20180709080554.21931-1-jhubbard@nvidia.com
    Proposed steps for fixing get_user_pages() + DMA problems.

help at all, or is it just too many references, and I should write the words
directly in the changelog?

Anyway, patch 3/3 is a just a working example (which we do want to submit, though), and
many more conversions will follow. But they don't have to be done all upfront--they
can be done in follow up patchsets. 

The put_user_page*() routines are, at this point, not going to significantly change
behavior. 

I'm working on an RFC that will show what the long-term fix to get_user_pages and
put_user_pages will look like. But meanwhile it's good to get started on converting
all of the call sites.

thanks,
-- 
John Hubbard
NVIDIA