linux-kernel - Re: [RFC PATCH 2/2] mm/gup: introduce vaddr_pin_pages

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Date:   Fri, 16 Aug 2019 10:47:21 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Jan Kara <jack@...e.cz>, John Hubbard <jhubbard@...dia.com>
Cc:     Ira Weiny <ira.weiny@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jérôme Glisse <jglisse@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-rdma@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] mm/gup: introduce vaddr_pin_pages_remote()

On 8/15/19 3:35 PM, Jan Kara wrote:
>> 
>> So when the GUP user uses MMU notifiers to stop writing to pages whenever
>> they are writeprotected with page_mkclean(), they don't really need page
>> pin - their access is then fully equivalent to any other mmap userspace
>> access and filesystem knows how to deal with those. I forgot out this case
>> when I wrote the above sentence.
>> 
>> So to sum up there are three cases:
>> 1) DIO case - GUP references to pages serving as DIO buffers are needed for
>>    relatively short time, no special synchronization with page_mkclean() or
>>    munmap() => needs FOLL_PIN
>> 2) RDMA case - GUP references to pages serving as DMA buffers needed for a
>>    long time, no special synchronization with page_mkclean() or munmap()
>>    => needs FOLL_PIN | FOLL_LONGTERM
>>    This case has also a special case when the pages are actually DAX. Then
>>    the caller additionally needs file lease and additional file_pin
>>    structure is used for tracking this usage.
>> 3) ODP case - GUP references to pages serving as DMA buffers, MMU notifiers
>>    used to synchronize with page_mkclean() and munmap() => normal page
>>    references are fine.

IMHO the munlock lesson told us about another one, that's in the end equivalent
to 3)

4) pinning for struct page manipulation only => normal page references are fine

> I want to add that I'd like to convert users in cases 1) and 2) from using
> GUP to using differently named function. Users in case 3) can stay as they
> are for now although ultimately I'd like to denote such use cases in a
> special way as well...

So after 1/2/3 is renamed/specially denoted, only 4) keeps the current interface?

> 								Honza
>