Re: [PATCH v4 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages()

From: Lorenzo Stoakes <lstoakes@gmail.com>
To: David Howells <dhowells@redhat.com>
Cc: David Hildenbrand <david@redhat.com>,
	Christoph Hellwig <hch@infradead.org>,
	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()
Date: Wed, 31 May 2023 15:10:53 +0100	[thread overview]
Message-ID: <0e307b3f-d367-4bb2-9506-93a3d70b3e97@lucifer.local> (raw)
In-Reply-To: <703628.1685541335@warthog.procyon.org.uk>

On Wed, May 31, 2023 at 02:55:35PM +0100, David Howells wrote:
> 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 used
> in other contexts too.  For that reason, I think FOLL_PIN was a poor choice of
> name:-/.  I guess the English language has got somewhat overloaded.  Maybe
> FOLL_PEG? ;-)
>

Might I suggest FOLL_FOLL? As we are essentially 'following' the page once
'pinned'. We could differentiate between what is currently FOLL_GET and
FOLL_PIN by using FOLL_FOLL and FOLL_FOLL_FOLL.

Patch series coming soon...

> > 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 pages will
> > >> not be incremented, and unpin_user_page() will similarly not decrement it."
> > > That's not really right (although it happens to be true), because we're
> > > talking primarily about the pin counter, not the refcount - and they may 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 count 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
>