linux-kernel - Re: [RFC 1/8] mm: add overflow and underflow checks for page->

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Date:   Tue, 26 Oct 2021 17:34:18 -0400
From:   Pasha Tatashin <pasha.tatashin@...een.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        linux-m68k@...ts.linux-m68k.org,
        Anshuman Khandual <anshuman.khandual@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        william.kucharski@...cle.com,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        schmitzmic@...il.com, Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, songmuchun@...edance.com,
        weixugc@...gle.com, Greg Thelen <gthelen@...gle.com>
Subject: Re: [RFC 1/8] mm: add overflow and underflow checks for page->_refcount

On Tue, Oct 26, 2021 at 3:50 PM Matthew Wilcox <willy@...radead.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.

Pasha