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

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Date:   Tue, 13 Aug 2019 14:08:57 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        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 Mon, Aug 12, 2019 at 05:07:32PM -0700, John Hubbard wrote:
> On 8/12/19 4:49 PM, Ira Weiny wrote:
> > On Sun, Aug 11, 2019 at 06:50:44PM -0700, john.hubbard@...il.com wrote:
> > > From: John Hubbard <jhubbard@...dia.com>
> ...
> > > diff --git a/drivers/infiniband/core/umem_odp.c b/drivers/infiniband/core/umem_odp.c
> > > index 53085896d718..fdff034a8a30 100644
> > > --- a/drivers/infiniband/core/umem_odp.c
> > > +++ b/drivers/infiniband/core/umem_odp.c
> > > @@ -534,7 +534,7 @@ static int ib_umem_odp_map_dma_single_page(
> > >   	}
> > >   out:
> > > -	put_user_page(page);
> > > +	vaddr_unpin_pages(&page, 1, &umem_odp->umem.vaddr_pin);
> > >   	if (remove_existing_mapping) {
> > >   		ib_umem_notifier_start_account(umem_odp);
> > > @@ -635,9 +635,10 @@ int ib_umem_odp_map_dma_pages(struct ib_umem_odp *umem_odp, u64 user_virt,
> > >   		 * complex (and doesn't gain us much performance in most use
> > >   		 * cases).
> > >   		 */
> > > -		npages = get_user_pages_remote(owning_process, owning_mm,
> > > +		npages = vaddr_pin_pages_remote(owning_process, owning_mm,
> > >   				user_virt, gup_num_pages,
> > > -				flags, local_page_list, NULL, NULL);
> > > +				flags, local_page_list, NULL, NULL,
> > > +				&umem_odp->umem.vaddr_pin);
> > 
> > Thinking about this part of the patch... is this pin really necessary?  This
> > code is not doing a long term pin.  The page just needs a reference while we
> > map it into the devices page tables.  Once that is done we should get notifiers
> > if anything changes and we can adjust.  right?
> > 
> 
> OK, now it's a little interesting: the FOLL_PIN is necessary, but maybe not
> FOLL_LONGTERM. Illustrating once again that it's actually necessary to allow
> these flags to vary independently.

Why is PIN necessary?  I think we do want all drivers to use the new
user_uaddr_vaddr_pin_user_pages() call...  :-P  But in this case I think a
simple "get" reference is enough to reference the page while we are using it.
If it changes after the "put/unpin" we get a fault which should handle the
change right?

The other issue I have with FOLL_PIN is what does it mean to call "...pin...()"
without FOLL_PIN?

This is another confusion of get_user_pages()...  you can actually call it
without FOLL_GET...  :-/  And you just don't get pages back.  I've never really
dug into how (or if) you "put" them later...

> 
> And that leads to another API refinement idea: let's set FOLL_PIN within the
> vaddr_pin_pages*() wrappers, and set FOLL_LONGTER in the *callers* of those
> wrappers, yes?

I've thought about this before and I think any default flags should simply
define what we want follow_pages to do.

Also, the addition of vaddr_pin information creates an implicit flag which if
not there disallows any file pages from being pinned.  It becomes our new
"longterm" flag.  FOLL_PIN _could_ be what we should use "internally".  But we
could also just use this implicit vaddr_pin flag and not add a new flag.

Finally, I struggle with converting everyone to a new call.  It is more
overhead to use vaddr_pin in the call above because now the GUP code is going
to associate a file pin object with that file when in ODP we don't need that
because the pages can move around.

This overhead may be fine, not sure in this case, but I don't see everyone
wanting it.

Ira