Re: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user mappings

[Peter Zijlstra]

On Mon, Oct 28, 2019 at 07:59:25PM +0000, Edgecombe, Rick P wrote:
> On Mon, 2019-10-28 at 14:55 +0100, Peter Zijlstra wrote:
> > On Mon, Oct 28, 2019 at 04:16:23PM +0300, Kirill A. Shutemov wrote:
> > 
> > > I think active use of this feature will lead to performance degradation of
> > > the system with time.
> > > 
> > > Setting a single 4k page non-present in the direct mapping will require
> > > splitting 2M or 1G page we usually map direct mapping with. And it's one
> > > way road. We don't have any mechanism to map the memory with huge page
> > > again after the application has freed the page.
> > 
> > Right, we recently had a 'bug' where ftrace triggered something like
> > this and facebook ran into it as a performance regression. So yes, this
> > is a real concern.
> 
> Don't e/cBPF filters also break the direct map down to 4k pages when calling
> set_memory_ro() on the filter for 64 bit x86 and arm?
> 
> I've been wondering if the page allocator should make some effort to find a
> broken down page for anything that can be known will have direct map permissions
> changed (or if it already groups them somehow). But also, why any potential
> slowdown of 4k pages on the direct map hasn't been noticed for apps that do a
> lot of insertions and removals of BPF filters, if this is indeed the case.

That should be limited to the module range. Random data maps could
shatter the world.