Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

Matthew Wilcox
2021-08-16T16:01:57Z

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.