Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

David Hildenbrand
2021-08-16T16:13:28Z

On 16.08.21 17:59, 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.

Oh okay, so it's actually much more complicated and complex than I
thought. Thanks for clarifying that! I recall virtiofsd had similar
requirements for sharing memory with the QEMU main process, I might be
wrong.

"existing shared memory area" and your initial page table example made
me assume that we are simply dealing with sharing page tables of MAP_SHARED.

It's actually something like a VMA container that you share between
processes. And whatever VMAs are currently inside that VMA container is
mirrored to other processes. I assume sharing page tables could actually
be an implementation detail, especially when keeping MAP_PRIVATE
(confusing in that context!) and other features that will give you
surprises (uffd) out of the picture.

--
Thanks,

David / dhildenb