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From:   Pasha Tatashin <pasha.tatashin@soleen.com>
Date:   Tue, 26 Oct 2021 21:21:29 -0400
Message-ID: <CA+CK2bBP_XJXN+wM=hmRO3C7x2Fi_SnbT_J0fU9xH0vnxU_Omg@mail.gmail.com>
Subject: Re: [RFC 1/8] mm: add overflow and underflow checks for page->_refcount
To:     Matthew Wilcox <willy@infradead.org>
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>,
        Muchun Song <songmuchun@bytedance.com>, weixugc@google.com,
        Greg Thelen <gthelen@google.com>
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On Tue, Oct 26, 2021 at 5:34 PM Pasha Tatashin
<pasha.tatashin@soleen.com> wrote:
>
> On Tue, Oct 26, 2021 at 3:50 PM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Tue, Oct 26, 2021 at 05:38:15PM +0000, Pasha Tatashin wrote:
> > >  static inline void page_ref_add(struct page *page, int nr)
> > >  {
> > > -     atomic_add(nr, &page->_refcount);
> > > +     int ret;
> > > +
> > > +     VM_BUG_ON(nr <= 0);
> > > +     ret = atomic_add_return(nr, &page->_refcount);
> > > +     VM_BUG_ON_PAGE(ret <= 0, page);
> >
> > This isn't right.  _refcount is allowed to overflow into the negatives.
> > See page_ref_zero_or_close_to_overflow() and the conversations that led
> > to it being added.
>
> #define page_ref_zero_or_close_to_overflow(page) \
> 1204   ((unsigned int) page_ref_count(page) + 127u <= 127u)
>
>
> Uh, right, I saw the macro but did not realize there was an (unsigned int) cast.
>
> OK, I think we can move this macro inside:
> include/linux/page_ref.h
>
> modify it to something like this:
> #define page_ref_zero_or_close_to_overflow(page) \
>    ((unsigned int) page_ref_count(page) + v + 127u <= v + 127u)
>
> The sub/dec can also be fixed to ensure that we do not underflow but
> still working with the fact that we use all 32bits of _refcount.

I think we can do that by using:
atomic_fetch_*() and check for overflow/underflow after operation. I
will send the updated series soon.

Pasha