linux-kernel - Re: [PATCH v4 1/3] mm: Don't pin ZERO_PAGE in pin_user

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Message-ID: <0e307b3f-d367-4bb2-9506-93a3d70b3e97@lucifer.local>
Date:   Wed, 31 May 2023 15:10:53 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     David Howells <dhowells@...hat.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>,
        Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Hillf Danton <hdanton@...a.com>,
        Christian Brauner <brauner@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v4 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages()

On Wed, May 31, 2023 at 02:55:35PM +0100, David Howells wrote:
> David Hildenbrand <david@...hat.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
>