Re: [PATCH v5 07/27] mm/mmap: Create a guard area between VMAs

[Andy Lutomirski]

On Thu, Oct 11, 2018 at 1:39 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
>
> On Thu, Oct 11, 2018 at 5:20 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
> > Create a guard area between VMAs to detect memory corruption.
> [...]
> > +config VM_AREA_GUARD
> > +       bool "VM area guard"
> > +       default n
> > +       help
> > +         Create a guard area between VM areas so that access beyond
> > +         limit can be detected.
> > +
> >  endmenu
>
> Sorry to bring this up so late, but Daniel Micay pointed out to me
> that, given that VMA guards will raise the number of VMAs by
> inhibiting vma_merge(), people are more likely to run into
> /proc/sys/vm/max_map_count (which limits the number of VMAs to ~65k by
> default, and can't easily be raised without risking an overflow of
> page->_mapcount on systems with over ~800GiB of RAM, see
> https://lore.kernel.org/lkml/20180208021112.GB14918@xxxxxxxxxxxxxxxxxxxxxx/
> and replies) with this change.
>
> Playing with glibc's memory allocator, it looks like glibc will use
> mmap() for 128KB allocations; so at 65530*128KB=8GB of memory usage in
> 128KB chunks, an application could run out of VMAs.

Ugh.

Do we have a free VM flag so we could do VM_GUARD to force a guard
page?  (And to make sure that, when a new VMA is allocated, it won't
be directly adjacent to a VM_GUARD VMA.)