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

From: Andy Lutomirski
Date: Thu Oct 11 2018 - 16:55:43 EST

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
> 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.


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.)