Hi Kirill, On Tue, Oct 07, 2014 at 02:10:26PM +0300, Kirill A. Shutemov wrote: > On Fri, Oct 03, 2014 at 07:08:00PM +0200, Andrea Arcangeli wrote: > > There's one constraint enforced to allow this simplification: the > > source pages passed to remap_anon_pages must be mapped only in one > > vma, but this is not a limitation when used to handle userland page > > faults with MADV_USERFAULT. The source addresses passed to > > remap_anon_pages should be set as VM_DONTCOPY with MADV_DONTFORK to > > avoid any risk of the mapcount of the pages increasing, if fork runs > > in parallel in another thread, before or while remap_anon_pages runs. > > Have you considered triggering COW instead of adding limitation on > pages' mapcount? The limitation looks artificial from interface POV. I haven't considered it, mostly because I see it as a feature that it returns -EBUSY. I prefer to avoid the risk of userland getting a successful retval but internally the kernel silently behaving non-zerocopy by mistake because some userland bug forgot to set MADV_DONTFORK on the src_vma. COW would be not zerocopy so it's not ok. We get sub 1msec latency for userfaults through 10gbit and we don't want to risk wasting CPU caches. I however considered allowing to extend the strict behavior (i.e. the feature) later in a backwards compatible way. We could provide a non-zerocopy beahvior with a RAP_ALLOW_COW flag that would then turn the -EBUSY error into a copy. It's also more complex to implement the cow now, so it would make the code that really matters, harder to review. So it may be preferable to extend this later in a backwards compatible way with a new RAP_ALLOW_COW flag. The current handling the flags is already written in a way that should allow backwards compatible extension with RAP_ALLOW_*: #define RAP_ALLOW_SRC_HOLES (1UL<<0) SYSCALL_DEFINE4(remap_anon_pages, unsigned long, dst_start, unsigned long, src_start, unsigned long, len, unsigned long, flags) [..] long err = -EINVAL; [..] if (flags & ~RAP_ALLOW_SRC_HOLES) return err; -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
