Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

David Hildenbrand
2021-08-16T12:43:20Z

On 16.08.21 14:20, 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?
>
>>
>> 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?

I found what you were referring to: CONFIG_ARCH_WANT_HUGE_PMD_SHARE

I was not aware that we have such a monstrosity in the kernel.

--
Thanks,

David / dhildenb