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

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Date:   Fri, 16 Aug 2019 18:13:55 +0200
From:   Jan Kara <jack@...e.cz>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     Jan Kara <jack@...e.cz>, Vlastimil Babka <vbabka@...e.cz>,
        John Hubbard <jhubbard@...dia.com>,
        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>,
        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 Fri 16-08-19 11:52:20, Jerome Glisse wrote:
> On Fri, Aug 16, 2019 at 05:44:04PM +0200, Jan Kara wrote:
> > On Fri 16-08-19 10:47:21, Vlastimil Babka wrote:
> > > 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
> > 
> > Right, it's good to have this for clarity.
> > 
> > > > 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?
> > 
> > Well, munlock() code doesn't even use GUP, just follow_page(). I'd wait to
> > see what's left after handling cases 1), 2), and 3) to decide about the
> > interface for the remainder.
> > 
> 
> For 3 we do not need to take a reference at all :) So just forget about 3
> it does not exist. For 3 the reference is the reference the CPU page table
> has on the page and that's it. GUP is no longer involve in ODP or anything
> like that.

Yes, I understand. But the fact is that GUP calls are currently still there
e.g. in ODP code. If you can make the code work without taking a page
reference at all, I'm only happy :)

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR