Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function

[Yu-cheng Yu]

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?

Yu-cheng