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.
