Re: [PATCH v1 00/14] Add support for shared PTEs across processes

Dave Hansen
2022-04-11T18:47:23Z

On 4/11/22 09:05, Khalid Aziz wrote:
> PTEs are shared at pgdir level and hence it imposes following
> requirements on the address and size given to the mshare():
>
> - Starting address must be aligned to pgdir size (512GB on x86_64).
> This alignment value can be looked up in /proc/sys/vm//mshare_size
> - Size must be a multiple of pgdir size
> - Any mappings created in this address range at any time become
> shared automatically
> - Shared address range can have unmapped addresses in it. Any access
> to unmapped address will result in SIGBUS
>
> 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.

What does this mean in practice?