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

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Message-ID: <20190815173237.GA30924@iweiny-DESK2.sc.intel.com>
Date:   Thu, 15 Aug 2019 10:32:37 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Jan Kara <jack@...e.cz>
Cc:     John Hubbard <jhubbard@...dia.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 Thu, Aug 15, 2019 at 03:35:10PM +0200, Jan Kara wrote:
> On Thu 15-08-19 15:26:22, Jan Kara wrote:
> > On Wed 14-08-19 20:01:07, John Hubbard wrote:
> > > On 8/14/19 5:02 PM, John Hubbard wrote:
> > > 
> > > Hold on, I *was* forgetting something: this was a two part thing, and
> > > you're conflating the two points, but they need to remain separate and
> > > distinct. There were:
> > > 
> > > 1. FOLL_PIN is necessary because the caller is clearly in the use case that
> > > requires it--however briefly they might be there. As Jan described it,
> > > 
> > > "Anything that gets page reference and then touches page data (e.g.
> > > direct IO) needs the new kind of tracking so that filesystem knows
> > > someone is messing with the page data." [1]
> > 
> > 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.
> 
> 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...
> 

Ok just to make this clear I threw up my current tree with your patches here:

https://github.com/weiny2/linux-kernel/commits/mmotm-rdmafsdax-b0-v4

I'm talking about dropping the final patch:
05fd2d3afa6b rdma/umem_odp: Use vaddr_pin_pages_remote() in ODP

The other 2 can stay.  I split out the *_remote() call.  We don't have a user
but I'll keep it around for a bit.

This tree is still WIP as I work through all the comments.  So I've not changed
names or variable types etc...  Just wanted to settle this.

Ira