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Date:   Sun, 19 Feb 2023 22:22:32 -0800
From:   Christoph Hellwig <hch@infradead.org>
To:     Jan Kara <jack@suse.cz>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Dave Chinner <david@fromorbit.com>,
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        linux-mm@kvack.org, John Hubbard <jhubbard@nvidia.com>,
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        David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH 4/5] block: Add support for bouncing pinned pages
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On Thu, Feb 16, 2023 at 01:33:16PM +0100, Jan Kara wrote:
> I'm a bit skeptical we can reasonably assess that (as much as I would love
> to just not write these pages and be done with it) because a lot of
> FOLL_LONGTERM users just pin passed userspace address range, then allow
> userspace to manipulate it with other operations, and finally unpin it with
> another call. Who knows whether shared pagecache pages are passed in and
> what userspace is doing with them while they are pinned? 

True.

So what other sensible thing could we do at a higher level?

Treat MAP_SHARED buffers that are long term registered as MAP_PRIVATE
while registered, and just do writeback using in-kernel O_DIRECT on
fsync?