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

[Pasha Tatashin]

On Tue, Dec 7, 2021 at 7:05 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Sat,  4 Dec 2021 18:23:13 +0000 Pasha Tatashin <pasha.tatashin@xxxxxxxxxx> 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