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? Are you referring to this? : 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.
