On Fri, Feb 12, 2021 at 02:51:17PM -0800, Axel Rasmussen wrote: > On Fri, Feb 12, 2021 at 2:44 PM Peter Xu <peterx@xxxxxxxxxx> wrote: > > > > On Fri, Feb 12, 2021 at 10:21:45PM +0000, Matthew Wilcox wrote: > > > On Thu, Feb 11, 2021 at 11:28:09AM -0800, Axel Rasmussen wrote: > > > > Ah, I had added this just after VM_UFFD_WP, without noticing that this > > > > would be sharing a bit with VM_LOCKED. That seems like not such a > > > > great idea. > > > > > > > > I don't see another unused bit, and I don't see some other obvious > > > > candidate to share with. So, the solution that comes to mind is > > > > > > it'd be even better if you didn't use the last unused bit for UFFD_WP. > > > not sure how feasible that is, but you can see we're really short on > > > bits here. > > > > UFFD_WP is used now for anonymouse already.. And the support for hugetlbfs and > > shmem is in rfc stage on the list. > > > > Is it possible to use CONFIG_ARCH_USES_HIGH_VMA_FLAGS here? So far uffd-wp is > > only working for 64 bit x86 too due to enlarged pte space. Maybe we can also > > let minor mode to only support 64 bit hosts. > > At least for my / Google's purposes, I don't care about 32-bit support > for this feature. I do care about both x86_64 and arm64, though. So > it's a possibility. > > Alternatively, the "it's an API feature not a registration mode" > approach I sent in my v6 also works for me, although it has some > drawbacks. Per-vma has finer granularity and logically more flexible. If it's low hanging fruit, let's think about it more before giving up so quickly. Sorry I commented late for this - I got diverged a bit in the past days. While you worked on it so fast (which in many cases still a good thing :). > > Another option is, would it be terrible to add an extra u16 or u32 for > UFFD flags to vm_area_struct (say within vm_userfaultfd_ctx)? > Historically we've already added a pointer, so maybe an extra say 16 > bits isn't so bad? This would avoid using *any* VM_* flags for UFFD, > even VM_UFFD_MISSING could be in this new flag field. For 64bit hosts there're still places for vm_flags. It's just 32bit, while there's option to make it 64bit-only. Even if we'd add a new field, those bits were still unused on 64bit hosts. IMHO we should try to use them before adding new field which will actually impact all hosts. Thanks, -- Peter Xu
