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

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Date:   Tue, 24 Jan 2023 11:06:49 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     David Howells <dhowells@...hat.com>
Cc:     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

On Tue, Jan 24, 2023 at 02:59:25PM +0000, David Howells wrote:
> 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.

Isn't that this:

if (cleanup_flags & PAGE_CLEANUP_NEEDED)
   gup_put_folio(folio, 1, cleanup_flags & PAGE_CLEANUP_UNPIN)


?

Three states - decr get, decr pinned, do nothing?

Jason