Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function
From: Yu-cheng Yu
Date: Mon Jun 10 2019 - 18:53:32 EST
On Mon, 2019-06-10 at 15:02 -0700, Dave Hansen wrote:
> On 6/10/19 1:58 PM, Yu-cheng Yu wrote:
> > > > On each memory request, the kernel then must consider a percentage of
> > > > allocated space in its calculation, and on systems with less memory
> > > > this quickly becomes a problem.
> > >
> > > I'm not sure what you're referring to here? Are you referring to our
> > > overcommit limits?
> > Yes.
> My assumption has always been that these large, potentially sparse
> hardware tables *must* be mmap()'d with MAP_NORESERVE specified. That
> should keep them from being problematic with respect to overcommit.
Ok, we will go back to do_mmap() with MAP_PRIVATE, MAP_NORESERVE and
VM_DONTDUMP. The bitmap will cover only 48-bit address space.
We then create PR_MARK_CODE_AS_LEGACY. The kernel will set the bitmap, but it
is going to be slow.
Perhaps we still let the app fill the bitmap?