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From:   David Howells <dhowells@redhat.com>
In-Reply-To: <Y8/rx6PQ4z9Tk8qQ@nvidia.com>
References: <Y8/rx6PQ4z9Tk8qQ@nvidia.com> <Y8/hhvfDtVcsgQd6@nvidia.com> <Y8/ZekMEAfi8VeFl@nvidia.com> <20230123173007.325544-1-dhowells@redhat.com> <20230123173007.325544-11-dhowells@redhat.com> <31f7d71d-0eb9-2250-78c0-2e8f31023c66@nvidia.com> <84721e8d-d40e-617c-b75e-ead51c3e1edf@nvidia.com> <852117.1674567983@warthog.procyon.org.uk> <852914.1674568628@warthog.procyon.org.uk> <859142.1674569510@warthog.procyon.org.uk> <864109.1674570473@warthog.procyon.org.uk>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     dhowells@redhat.com, John Hubbard <jhubbard@nvidia.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        Matthew Wilcox <willy@infradead.org>,
        Jens Axboe <axboe@kernel.dk>, Jan Kara <jack@suse.cz>,
        Jeff Layton <jlayton@kernel.org>,
        Logan Gunthorpe <logang@deltatee.com>,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
        linux-mm@kvack.org
Subject: Re: [PATCH v8 10/10] mm: Renumber FOLL_PIN and FOLL_GET down
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Date:   Tue, 24 Jan 2023 14:59:25 +0000
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Jason Gunthorpe <jgg@nvidia.com> wrote:

> What is the 3rd state?

Consider a network filesystem message generated for a direct I/O that the
network filesystem does zerocopy on.  You may have an sk_buff that has
fragments from one or more of three different sources:

 (1) Fragments consisting of specifically allocated pages, such as the
     IP/UDP/TCP headers that have refs taken on them.

 (2) Fragments consisting of zerocopy kernel buffers that has neither refs nor
     pins belonging to the sk_buff.

     iov_iter_extract_pages() will not take pins when extracting from, say, an
     XARRAY-type or KVEC-type iterator.  iov_iter_extract_mode() will return
     0.

 (3) Fragments consisting of zerocopy user buffers that have pins taken on
     them belonging to the sk_buff.

     iov_iter_extract_pages() will take pins when extracting from, say, a
     UBUF-type or IOVEC-type iterator.  iov_iter_extract_mode() will return
     FOLL_PIN (at the moment).

So you have three states: Ref'd, pinned and no-retention.

David