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.
