Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

Matthew Wilcox
2021-08-16T14:00:31Z

On Mon, Aug 16, 2021 at 03:24:38PM +0200, David Hildenbrand wrote:
> On 16.08.21 14:46, Matthew Wilcox wrote:
> > 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.
>
> I read that somewhere else before, yet we have heavy 1 GiB page users,
> especially in the context of VMs and DPDK.

I wonder if those users actually benchmarked. Or whether the memory
savings worked out so well for them that the loss of TLB performance
didn't matter.

> So, it only works for hugetlbfs in case uffd is not in place (-> no
> per-process data in the page table) and we have an actual shared mappings.
> When unsharing, we zap the PUD entry, which will result in allocating a
> per-process page table on next fault.

I think uffd was a huge mistake. It should have been a filesystem
instead of a hack on the side of anonymous memory.

> I will rephrase my previous statement "hugetlbfs just doesn't raise these
> problems because we are special casing it all over the place already". For
> example, not allowing to swap such pages. Disallowing MADV_DONTNEED. Special
> hugetlbfs locking.

Sure, that's why I want to drag this feature out of "oh this is a
hugetlb special case" and into "this is something Linux supports".