Re: [PATCH v2 3/5] mm: madvise: implement lightweight guard page mechanism

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Mon, Oct 21, 2024 at 07:05:27PM +0200, David Hildenbrand wrote:
> On 20.10.24 18:20, Lorenzo Stoakes wrote:
> > Implement a new lightweight guard page feature, that is regions of userland
> > virtual memory that, when accessed, cause a fatal signal to arise.
> >
> > Currently users must establish PROT_NONE ranges to achieve this.
> >
> > However this is very costly memory-wise - we need a VMA for each and every
> > one of these regions AND they become unmergeable with surrounding VMAs.
> >
> > In addition repeated mmap() calls require repeated kernel context switches
> > and contention of the mmap lock to install these ranges, potentially also
> > having to unmap memory if installed over existing ranges.
> >
> > The lightweight guard approach eliminates the VMA cost altogether - rather
> > than establishing a PROT_NONE VMA, it operates at the level of page table
> > entries - poisoning PTEs such that accesses to them cause a fault followed
> > by a SIGSGEV signal being raised.
> >
> > This is achieved through the PTE marker mechanism, which a previous commit
> > in this series extended to permit this to be done, installed via the
> > generic page walking logic, also extended by a prior commit for this
> > purpose.
> >
> > These poison ranges are established with MADV_GUARD_POISON, and if the
> > range in which they are installed contain any existing mappings, they will
> > be zapped, i.e. free the range and unmap memory (thus mimicking the
> > behaviour of MADV_DONTNEED in this respect).
> >
> > Any existing poison entries will be left untouched. There is no nesting of
> > poisoned pages.
> >
> > Poisoned ranges are NOT cleared by MADV_DONTNEED, as this would be rather
> > unexpected behaviour, but are cleared on process teardown or unmapping of
> > memory ranges.
> >
> > Ranges can have the poison property removed by MADV_GUARD_UNPOISON -
> > 'remedying' the poisoning. The ranges over which this is applied, should
> > they contain non-poison entries, will be untouched, only poison entries
> > will be cleared.
> >
> > We permit this operation on anonymous memory only, and only VMAs which are
> > non-special, non-huge and not mlock()'d (if we permitted this we'd have to
> > drop locked pages which would be rather counterintuitive).
> >
> > Suggested-by: Vlastimil Babka <vbabka@suse.cz>
> > Suggested-by: Jann Horn <jannh@google.com>
> > Suggested-by: David Hildenbrand <david@redhat.com>
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> > ---
> >   arch/alpha/include/uapi/asm/mman.h     |   3 +
> >   arch/mips/include/uapi/asm/mman.h      |   3 +
> >   arch/parisc/include/uapi/asm/mman.h    |   3 +
> >   arch/xtensa/include/uapi/asm/mman.h    |   3 +
> >   include/uapi/asm-generic/mman-common.h |   3 +
> >   mm/madvise.c                           | 168 +++++++++++++++++++++++++
> >   mm/mprotect.c                          |   3 +-
> >   mm/mseal.c                             |   1 +
> >   8 files changed, 186 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/alpha/include/uapi/asm/mman.h b/arch/alpha/include/uapi/asm/mman.h
> > index 763929e814e9..71e13f27742d 100644
> > --- a/arch/alpha/include/uapi/asm/mman.h
> > +++ b/arch/alpha/include/uapi/asm/mman.h
> > @@ -78,6 +78,9 @@
> >   #define MADV_COLLAPSE	25		/* Synchronous hugepage collapse */
> > +#define MADV_GUARD_POISON 102		/* fatal signal on access to range */
> > +#define MADV_GUARD_UNPOISON 103		/* revoke guard poisoning */
>
> Just to raise it here: MADV_GUARD_INSTALL / MADV_GUARD_REMOVE or sth. like
> that would have been even clearer, at least to me.

:)

It still feels like poisoning to me because we're explicitly putting
something in the page tables to make a range have different fault behaviour
like a HW poisoning, and 'installing' suggests backing or something like
this, I think that's more confusing.

>
> But no strong opinion, just if somebody else reading along was wondering
> about the same.
>
>
> I'm hoping to find more time to have a closer look at this this week, but in
> general, the concept sounds reasonable to me.

Thanks!

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