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Date:   Tue, 26 Oct 2021 21:13:33 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Pasha Tatashin <pasha.tatashin@soleen.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>,
        linux-m68k@lists.linux-m68k.org,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        william.kucharski@oracle.com,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        schmitzmic@gmail.com, Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <guro@fb.com>, songmuchun@bytedance.com,
        weixugc@google.com, Greg Thelen <gthelen@google.com>
Subject: Re: [RFC 0/8] Hardening page _refcount
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References: <20211026173822.502506-1-pasha.tatashin@soleen.com>
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On Tue, Oct 26, 2021 at 02:30:25PM -0400, Pasha Tatashin wrote:
> On Tue, Oct 26, 2021 at 2:24 PM Matthew Wilcox <willy@infradead.org> wrote:
> > I think this is overkill.  Won't we get exactly the same protection
> > by simply testing that page->_refcount == 0 in set_page_count()?
> > Anything which triggers that BUG_ON would already be buggy because
> > it can race with speculative gets.
> 
> We can't because set_page_count(v) is used for
> 1. changing _refcount form a current value to unconstrained v
> 2.  initialize _refcount from undefined state to v.
> 
> In this work we forbid the first case, and reduce the second case to
> initialize only to 1.

Anything that is calling set_page_refcount() on something which is
not 0 is buggy today.  There are several ways to increment the page
refcount speculatively if it is not 0.  eg lockless GUP and page cache
reads.  So we could have:

CPU 0: alloc_page() (refcount now 1)
CPU 1: get_page_unless_zero() (refcount now 2)
CPU 0: set_page_refcount(5) (refcount now 5)
CPU 1: put_page() (refcount now 4)

Now the refcount is wrong.  So it is *only* safe to call
set_page_refcount() if the refcount is 0.  If you can find somewhere
that's calling set_page_refcount() on a non-0 refcount, that's a bug
that needs to be fixed.