linux-kernel - Re: [PATCH v1 05/18] mm: improve folio_likely_mapped_shared() using the mapcount of large folios

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAK1f24kph=hV6Da=fAXFuKChx5JVasPLYC7efQbPr+JT0WxQ1A@mail.gmail.com>
Date: Tue, 16 Apr 2024 18:52:24 +0800
From: Lance Yang <ioworker0@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: akpm@...ux-foundation.org, cgroups@...r.kernel.org, chris@...kel.net, 
	corbet@....net, dalias@...c.org, fengwei.yin@...el.com, 
	glaubitz@...sik.fu-berlin.de, hughd@...gle.com, jcmvbkbc@...il.com, 
	linmiaohe@...wei.com, linux-doc@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, linux-sh@...r.kernel.org, 
	linux-trace-kernel@...r.kernel.org, muchun.song@...ux.dev, 
	naoya.horiguchi@....com, peterx@...hat.com, richardycc@...gle.com, 
	ryan.roberts@....com, shy828301@...il.com, willy@...radead.org, 
	ysato@...rs.sourceforge.jp, ziy@...dia.com
Subject: Re: [PATCH v1 05/18] mm: improve folio_likely_mapped_shared() using
 the mapcount of large folios

On Tue, Apr 16, 2024 at 6:47 PM David Hildenbrand <david@...hat.com> wrote:
>
> On 16.04.24 12:40, Lance Yang wrote:
> > Hey David,
> >
> > Maybe I spotted a bug below.
>
> Thanks for the review!
>
> >
> > [...]
> >   static inline bool folio_likely_mapped_shared(struct folio *folio)
> >   {
> > -     return page_mapcount(folio_page(folio, 0)) > 1;
> > +     int mapcount = folio_mapcount(folio);
> > +
> > +     /* Only partially-mappable folios require more care. */
> > +     if (!folio_test_large(folio) || unlikely(folio_test_hugetlb(folio)))
> > +             return mapcount > 1;
> > +
> > +     /* A single mapping implies "mapped exclusively". */
> > +     if (mapcount <= 1)
> > +             return false;
> > +
> > +     /* If any page is mapped more than once we treat it "mapped shared". */
> > +     if (folio_entire_mapcount(folio) || mapcount > folio_nr_pages(folio))
> > +             return true;
> >
> > bug: if a PMD-mapped THP is exclusively mapped, the folio_entire_mapcount()
> > function will return 1 (atomic_read(&folio->_entire_mapcount) + 1).
>
> If it's exclusively mapped, then folio_mapcount(folio)==1. In which case
> the previous statement:
>
> if (mapcount <= 1)
>         return false;
>
> Catches it.

You're right!

>
> IOW, once we reach this point we now that folio_mapcount(folio) > 1, and
> there must be something else besides the entire mapping ("more than once").
>
>
> Or did I not address your concern?

Sorry, my mistake :(

Thanks,
Lance

>
> --
> Cheers,
>
> David / dhildenb
>