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

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Date:   Fri, 16 Aug 2019 14:59:54 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Jan Kara <jack@...e.cz>, 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 Fri, Aug 16, 2019 at 11:50:09AM -0700, John Hubbard wrote:
> On 8/16/19 11:33 AM, Ira Weiny wrote:
> > On Fri, Aug 16, 2019 at 05:41:08PM +0200, Jan Kara wrote:
> > > On Thu 15-08-19 19:14:08, John Hubbard wrote:
> > > > On 8/15/19 10:41 AM, John Hubbard wrote:
> > > > > On 8/15/19 10:32 AM, Ira Weiny wrote:
> > > > > > 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:
> > > > ...
> > > > 
> > > > OK, there was only process_vm_access.c, plus (sort of) Bharath's sgi-gru
> > > > patch, maybe eventually [1].  But looking at process_vm_access.c, I think
> > > > it is one of the patches that is no longer applicable, and I can just
> > > > drop it entirely...I'd welcome a second opinion on that...
> > > 
> > > I don't think you can drop the patch. process_vm_rw_pages() clearly touches
> > > page contents and does not synchronize with page_mkclean(). So it is case
> > > 1) and needs FOLL_PIN semantics.
> > 
> > John could you send a formal patch using vaddr_pin* and I'll add it to the
> > tree?
> > 
> 
> Yes...hints about which struct file to use here are very welcome, btw. This part
> of mm is fairly new to me.

I'm still working out the final semantics of vaddr_pin*.  But right now you
don't need a vaddr_pin if you don't specify FOLL_LONGTERM.

Since case 1, this case, does not need FOLL_LONGTERM I think it is safe to
simply pass NULL here.

OTOH we could just track this against the mm_struct.  But I don't think we need
to because this pin should be transient.

And this is why I keep leaning toward _not_ putting these flags in the
vaddr_pin*() calls.  I know this is what I did but I think I'm wrong.  It should
be the caller specifying what they want and the vaddr_pin*() calls check that
what they are asking for is correct.

Ira

> 
> thanks,
> -- 
> John Hubbard
> NVIDIA