netdev - Re: Phyr Starter

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Message-ID: <20220111150222.GM2328285@nvidia.com>
Date:   Tue, 11 Jan 2022 11:02:22 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     John Hubbard <jhubbard@...dia.com>, linux-kernel@...r.kernel.org,
        Christoph Hellwig <hch@....de>,
        Joao Martins <joao.m.martins@...cle.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Ming Lei <ming.lei@...hat.com>, linux-block@...r.kernel.org,
        netdev@...r.kernel.org, linux-mm@...ck.org,
        linux-rdma@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        nvdimm@...ts.linux.dev
Subject: Re: Phyr Starter

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