linux-kernel - Re: [PATCH v8 10/10] mm: Renumber FOLL_PIN and FOLL

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Message-ID: <875206.1674572365@warthog.procyon.org.uk>
Date:   Tue, 24 Jan 2023 14:59:25 +0000
From:   David Howells <dhowells@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     dhowells@...hat.com, John Hubbard <jhubbard@...dia.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
        linux-mm@...ck.org
Subject: Re: [PATCH v8 10/10] mm: Renumber FOLL_PIN and FOLL_GET down

Jason Gunthorpe <jgg@...dia.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