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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 09:35:52 +0100
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David Hildenbrand <david@redhat.com> wrote:

> > Make pin_user_pages*() leave a ZERO_PAGE unpinned if it extracts a poi=
nter
> > to it from the page tables and make unpin_user_page*() correspondingly
> > ignore a ZERO_PAGE when unpinning.  We don't want to risk overrunning =
a
> > zero page's refcount as we're only allowed ~2 million pins on it -
> > something that userspace can conceivably trigger.
> =

> 2 millions pins (FOLL_PIN, which increments the refcount by 1024) or 2 m=
illion
> references ?

Definitely pins.  It's tricky because we've been using "pinned" to mean he=
ld
by a refcount or held by a flag too.

2 million pins on the zero page is in the realms of possibility.  It only
takes 32768 64-page DIO writes.

> > @@ -3079,6 +3096,9 @@ EXPORT_SYMBOL_GPL(get_user_pages_fast);
> >    *
> >    * FOLL_PIN means that the pages must be released via unpin_user_pag=
e(). Please
> >    * see Documentation/core-api/pin_user_pages.rst for further details=
.
> > + *
> > + * Note that if a zero_page is amongst the returned pages, it will no=
t have
> > + * pins in it and unpin_user_page() will not remove pins from it.
> >    */
> =

> "it will not have pins in it" sounds fairly weird to a non-native speake=
r.

Oh, I know.  The problem is that "pin" is now really ambiguous.  Can we ch=
ange
"FOLL_PIN" to "FOLL_NAIL"?  Or maybe "FOLL_SCREW" - your pages are screwed=
 if
you use DIO and fork at the same time.

> "Note that the refcount of any zero_pages returned among the pinned page=
s will
> not be incremented, and unpin_user_page() will similarly not decrement i=
t."

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 b=
e
separate.

David