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Date:   Tue, 19 Oct 2021 14:37:10 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Kent Overstreet <kent.overstreet@gmail.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Matthew Wilcox <willy@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        "Darrick J. Wong" <djwong@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        David Howells <dhowells@redhat.com>
Subject: Re: Splitting struct page into multiple types - Was: re: Folio
 discussion recap -
Message-ID: <20211019173710.GI3686969@ziepe.ca>
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On Tue, Oct 19, 2021 at 12:11:35PM -0400, Kent Overstreet wrote:

> I have no idea if this approach works for network pool pages or how those would
> be used, I haven't gotten that far - if someone can chime in about those that

Generally the driver goal is to create a shared memory buffer between
kernel and user space.

The broadly two common patterns are to have userspace call mmap() and
the kernel side returns the kernel pages from there - getting them
from some kernel allocator.

Or, userspace allocates the buffer and the kernel driver does
pin_user_pages() to import them to its address space.

I think it is quite feasible to provide some simple library API to
manage the shared buffer through mmap approach, and if that library
wants to allocate inodes, folios and what not it should be possible.

It would help this idea to see Christoph's cleanup series go forward:

https://lore.kernel.org/all/20200508153634.249933-1-hch@lst.de/

As it makes it alot easier for drivers to get inodes in the first
place.

> would be great. But, the end goal I'm envisioning is a world where _only_ bog
> standard file & anonymous pages are mapped to userspace - then _mapcount can be
> deleted from struct page and only needs to live in struct folio.

There is a lot of work in the past years on ZONE_DEVICE pages into
userspace. Today FSDAX is kind of a mashup of a file and device page,
but other stuff is less obvious, especially DEVICE_COHERENT.

Jason