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From:   David Howells <dhowells@redhat.com>
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To:     David Hildenbrand <david@redhat.com>
Cc:     dhowells@redhat.com, Christoph Hellwig <hch@infradead.org>,
        Lorenzo Stoakes <lstoakes@gmail.com>,
        Jens Axboe <axboe@kernel.dk>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
        Jeff Layton <jlayton@kernel.org>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        Hillf Danton <hdanton@sina.com>,
        Christian Brauner <brauner@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v4 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages()
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Date:   Wed, 31 May 2023 14:55:35 +0100
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David Hildenbrand <david@redhat.com> wrote:

> Yes, it would be clearer if we would be using "pinned" now only for FOLL=
_PIN

You're not likely to get that.  "To pin" is too useful a verb that gets us=
ed
in other contexts too.  For that reason, I think FOLL_PIN was a poor choic=
e of
name:-/.  I guess the English language has got somewhat overloaded.  Maybe
FOLL_PEG? ;-)

> and everything else is simply "taking a temporary reference on the page"=
.

Excluding refs taken with pins, many refs are more permanent than pins as,=
 so
far as I'm aware, pins only last for the duration of an I/O operation.

> >> "Note that the refcount of any zero_pages returned among the pinned p=
ages will
> >> not be incremented, and unpin_user_page() will similarly not decremen=
t it."
> > That's not really right (although it happens to be true), because we'r=
e
> > talking primarily about the pin counter, not the refcount - and they m=
ay be
> > separate.
> =

> In any case (FOLL_PIN/FOLL_GET) you increment/decrement the refcount. If=
 we
> have a separate pincount, we increment/decrement the refcount by 1 when
> (un)pinning.

FOLL_GET isn't relevant here - only FOLL_PIN.  Yes, as it happens, we coun=
t a
ref if we count a pin, but that's kind of irrelevant; what matters is that=
 the
effect must be undone with un-PUP.

It would be nice not to get a ref on the zero page in FOLL_GET, but I don'=
t
think we can do that yet.  Too many places assume that GUP will give them =
a
ref they can release later via ordinary methods.

David