Re: Phyr Starter

From: Jason Gunthorpe <jgg@nvidia.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: John Hubbard <jhubbard@nvidia.com>,
	linux-kernel@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
	Joao Martins <joao.m.martins@oracle.com>,
	Logan Gunthorpe <logang@deltatee.com>,
	Ming Lei <ming.lei@redhat.com>,
	linux-block@vger.kernel.org, netdev@vger.kernel.org,
	linux-mm@kvack.org, linux-rdma@vger.kernel.org,
	dri-devel@lists.freedesktop.org, nvdimm@lists.linux.dev
Subject: Re: Phyr Starter
Date: Tue, 11 Jan 2022 11:02:22 -0400	[thread overview]
Message-ID: <20220111150222.GM2328285@nvidia.com> (raw)
In-Reply-To: <Yd2NraXh3ka8PdrQ@casper.infradead.org>

On Tue, Jan 11, 2022 at 02:01:17PM +0000, Matthew Wilcox wrote:
> On Tue, Jan 11, 2022 at 12:17:18AM -0800, John Hubbard wrote:
> > Zooming in on the pinning aspect for a moment: last time I attempted to
> > convert O_DIRECT callers from gup to pup, I recall wanting very much to
> > record, in each bio_vec, whether these pages were acquired via FOLL_PIN,
> > or some non-FOLL_PIN method. Because at the end of the IO, it is not
> > easy to disentangle which pages require put_page() and which require
> > unpin_user_page*().
> > 
> > And changing the bio_vec for *that* purpose was not really acceptable.
> > 
> > But now that you're looking to change it in a big way (and with some
> > spare bits avaiable...oohh!), maybe I can go that direction after all.
> > 
> > Or, are you looking at a design in which any phyr is implicitly FOLL_PIN'd
> > if it exists at all?
> 
> That.  I think there's still good reasons to keep a single-page (or
> maybe dual-page) GUP around, but no reason to mix it with ranges.
> 
> > Or any other thoughts in this area are very welcome.
> 
> That's there's no support for unpinning part of a range.  You pin it,
> do the IO, unpin it.  That simplifies the accounting.

VFIO wouldn't like this :(

Jason