linux-kernel - Re: [PATCH v2 3/4] mm: page table check

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Date:   Wed, 8 Dec 2021 12:35:02 -0500
From:   Pasha Tatashin <pasha.tatashin@...een.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        David Rientjes <rientjes@...gle.com>,
        Paul Turner <pjt@...gle.com>, weixugc@...gle.com,
        Greg Thelen <gthelen@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Will Deacon <will@...nel.org>, Mike Rapoport <rppt@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>, masahiroy@...nel.org,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        frederic@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        Muchun Song <songmuchun@...edance.com>,
        Fusion Future <qydwhotmail@...il.com>
Subject: Re: [PATCH v2 3/4] mm: page table check

On Tue, Dec 7, 2021 at 7:05 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Sat,  4 Dec 2021 18:23:13 +0000 Pasha Tatashin <pasha.tatashin@...een.com> wrote:
>
> > Check user page table entries at the time they are added and removed.
> >
> > Allows to synchronously catch memory corruption issues related to double
> > mapping.
> >
> > When a pte for an anonymous page is added into page table, we verify that
> > this pte does not already point to a file backed page, and vice versa if
> > this is a file backed page that is being added we verify that this page
> > does not have an anonymous mapping
> >
> > We also enforce that read-only sharing for anonymous pages is allowed
> > (i.e.  cow after fork).  All other sharing must be for file pages.
> >
> > Page table check allows to protect and debug cases where "struct page"
> > metadata became corrupted for some reason.  For example, when refcnt or
> > mapcount become invalid.
> >
> > ...
> >
> > --- a/arch/Kconfig
> > +++ b/arch/Kconfig
> > @@ -1307,6 +1307,9 @@ config HAVE_ARCH_PFN_VALID
> >  config ARCH_SUPPORTS_DEBUG_PAGEALLOC
> >       bool
> >
> > +config ARCH_SUPPORTS_PAGE_TABLE_CHECK
> > +     bool
> > +
>
> I guess a dependency on CONFIG_DEBUG_VM would be appropriate?

I do not think CONFIG_DEBUG_VM is needed here. We would like to have
page table check enabled on some production machines to ensure there
are no double mappings. With CONFIG_DEBUG_VM enabled that would not be
possible. For example, CONFIG_PAGE_OWNER, also uses extended struct
page entries and does not depend on CONFIG_DEBUG_VM

Pasha