On Mon, Aug 16, 2021 at 02:20:43PM +0200, David Hildenbrand wrote: > On 16.08.21 14:07, Matthew Wilcox wrote: > > On Mon, Aug 16, 2021 at 10:02:22AM +0200, David Hildenbrand wrote: > > > > Mappings within this address range behave as if they were shared > > > > between threads, so a write to a MAP_PRIVATE mapping will create a > > > > page which is shared between all the sharers. The first process that > > > > declares an address range mshare'd can continue to map objects in the > > > > shared area. All other processes that want mshare'd access to this > > > > memory area can do so by calling mshare(). After this call, the > > > > address range given by mshare becomes a shared range in its address > > > > space. Anonymous mappings will be shared and not COWed. > > > > > > Did I understand correctly that you want to share actual page tables between > > > processes and consequently different MMs? That sounds like a very bad idea. > > > > That is the entire point. Consider a machine with 10,000 instances > > of an application running (process model, not thread model). If each > > application wants to map 1TB of RAM using 2MB pages, that's 4MB of page > > tables per process or 40GB of RAM for the whole machine. > > What speaks against 1 GB pages then? Until recently, the CPUs only having 4 1GB TLB entries. I'm sure we still have customers using that generation of CPUs. 2MB pages perform better than 1GB pages on the previous generation of hardware, and I haven't seen numbers for the next generation yet. > > There's a reason hugetlbfs was enhanced to allow this page table sharing. > > I'm not a fan of the implementation as it gets some locks upside down, > > so this is an attempt to generalise the concept beyond hugetlbfs. > > Who do we account the page tables to? What are MADV_DONTNEED semantics? Who > cleans up the page tables? What happens during munmap? How does the rmap > even work? How to we actually synchronize page table walkers? > > See how hugetlbfs just doesn't raise these problems because we are sharing > pages and not page tables? No, really, hugetlbfs shares page tables already. You just didn't notice that yet. > > Think of it like partial threading. You get to share some parts, but not > > all, of your address space with your fellow processes. Obviously you > > don't want to expose this to random other processes, only to other > > instances of yourself being run as the same user. > > Sounds like a nice way to over-complicate MM to optimize for some special > use cases. I know, I'm probably wrong. :) It's really not as bad as you seem to think it is.
