linux-kernel - Re: [PATCH v3 1/9] mm: add overflow and underflow checks for page->

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Date:   Thu, 27 Jan 2022 14:42:09 -0500
From:   Pasha Tatashin <pasha.tatashin@...een.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        linux-m68k@...ts.linux-m68k.org,
        Anshuman Khandual <anshuman.khandual@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        william.kucharski@...cle.com,
        Mike Kravetz <mike.kravetz@...cle.com>,
        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>,
        Muchun Song <songmuchun@...edance.com>,
        Wei Xu <weixugc@...gle.com>, Greg Thelen <gthelen@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Paul Turner <pjt@...gle.com>, Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH v3 1/9] mm: add overflow and underflow checks for page->_refcount

> > diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
> > index 2e677e6ad09f..fe4864f7f69c 100644
> > --- a/include/linux/page_ref.h
> > +++ b/include/linux/page_ref.h
> > @@ -117,7 +117,10 @@ static inline void init_page_count(struct page *page)
> >
> >  static inline void page_ref_add(struct page *page, int nr)
> >  {
> > -     atomic_add(nr, &page->_refcount);
> > +     int old_val = atomic_fetch_add(nr, &page->_refcount);
> > +     int new_val = old_val + nr;
> > +
> > +     VM_BUG_ON_PAGE((unsigned int)new_val < (unsigned int)old_val, page);
>
> This seems somewhat weird, as it will trigger not just on overflow, but also
> if nr is negative. Which I think is valid usage, even though the function
> has 'add' in name, because 'nr' is signed?

I have not found any places in the mainline kernel where nr is
negative in page_ref_add(). I think, by adding this assert we ensure
that when 'add' shows up in backtraces it can be assured that the ref
count has increased, and if page_ref_sub() showed up it means it
decreased. It is strange to have both functions, and yet allow them to
do the opposite. We can also change the type to unsigned.

Pasha