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Date:   Tue, 11 Jan 2022 14:01:17 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     linux-kernel@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Joao Martins <joao.m.martins@oracle.com>,
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        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
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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.