Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

[Khalid Aziz]

On 8/16/21 9:59 AM, Matthew Wilcox wrote:
> On Mon, Aug 16, 2021 at 05:01:44PM +0200, David Hildenbrand wrote:
> > On 16.08.21 16:40, Matthew Wilcox wrote:
> > > On Mon, Aug 16, 2021 at 04:33:09PM +0200, David Hildenbrand wrote:
> > > > > > I did not follow why we have to play games with MAP_PRIVATE, and having
> > > > > > private anonymous pages shared between processes that don't COW, introducing
> > > > > > new syscalls etc.
> > > > >
> > > > > It's not about SHMEM, it's about file-backed pages on regular
> > > > > filesystems.  I don't want to have XFS, ext4 and btrfs all with their
> > > > > own implementations of ARCH_WANT_HUGE_PMD_SHARE.
> > > >
> > > > Let me ask this way: why do we have to play such games with MAP_PRIVATE?
> > >
> > > : 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.
> > >
> > > If so, that's a misunderstanding, because there are no games being played.
> > > What Khalid's saying there is that because the page tables are already
> > > shared for that range of address space, the COW of a MAP_PRIVATE will
> > > create a new page, but that page will be shared between all the sharers.
> > > The second write to a MAP_PRIVATE page (by any of the sharers) will not
> > > create a COW situation.  Just like if all the sharers were threads of
> > > the same process.
> >
> > It actually seems to be just like I understood it. We'll have multiple
> > processes share anonymous pages writable, even though they are not using
> > shared memory.
> >
> > IMHO, sharing page tables to optimize for something kernel-internal (page
> > table consumption) should be completely transparent to user space. Just like
> > ARCH_WANT_HUGE_PMD_SHARE currently is unless I am missing something
> > important.
> >
> > The VM_MAYSHARE check in want_pmd_share()->vma_shareable() makes me assume
> > that we really only optimize for MAP_SHARED right now, never for
> > MAP_PRIVATE.
>
> It's definitely *not* about being transparent to userspace.  It's about
> giving userspace new functionality where multiple processes can choose
> to share a portion of their address space with each other.  What any
> process changes in that range changes, every sharing process sees.
> mmap(), munmap(), mprotect(), mremap(), everything.

Exactly and to further elaborate, once a process calls mshare() to declare its intent to share PTEs for a range of 
address and another process accepts that sharing by calling mshare() itself, the two (or more) processes have agreed to 
share PTEs for that entire address range. A MAP_PRIVATE mapping in this address range goes against the original intent 
of sharing and what we are saying is the original intent of sharing takes precedence in case of this conflict.

--
Khalid