Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

[Matthew Wilcox]

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.