On Tue, Jan 12, 2021 at 11:15:43AM -0800, Nadav Amit wrote: > > On Jan 12, 2021, at 11:02 AM, Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx> wrote: > > > > Le 12/01/2021 à 17:57, Peter Zijlstra a écrit : > >> On Tue, Jan 12, 2021 at 04:47:17PM +0100, Laurent Dufour wrote: > >>> Le 12/01/2021 à 12:43, Vinayak Menon a écrit : > >>>> Possibility of race against other PTE modifiers > >>>> > >>>> 1) Fork - We have seen a case of SPF racing with fork marking PTEs RO and that > >>>> is described and fixed here https://lore.kernel.org/patchwork/patch/1062672/ > >> Right, that's exactly the kind of thing I was worried about. > >>>> 2) mprotect - change_protection in mprotect which does the deferred flush is > >>>> marked under vm_write_begin/vm_write_end, thus SPF bails out on faults > >>>> on those VMAs. > >> Sure, mprotect also changes vm_flags, so it really needs that anyway. > >>>> 3) userfaultfd - mwriteprotect_range is not protected unlike in (2) above. > >>>> But SPF does not take UFFD faults. > >>>> 4) hugetlb - hugetlb_change_protection - called from mprotect and covered by > >>>> (2) above. > >>>> 5) Concurrent faults - SPF does not handle all faults. Only anon page faults. > >> What happened to shared/file-backed stuff? ISTR I had that working. > > > > File-backed mappings are not processed in a speculative way, there were options to manage some of them depending on the underlying file system but that's still not done. > > > > Shared anonymous mapping, are also not yet handled in a speculative way (vm_ops is not null). > > > >>>> Of which do_anonymous_page and do_swap_page are NONE/NON-PRESENT->PRESENT > >>>> transitions without tlb flush. And I hope do_wp_page with RO->RW is fine as well. > >> The tricky one is demotion, specifically write to non-write. > >>>> I could not see a case where speculative path cannot see a PTE update done via > >>>> a fault on another CPU. > >> One you didn't mention is the NUMA balancing scanning crud; although I > >> think that's fine, loosing a PTE update there is harmless. But I've not > >> thought overly hard on it. > > > > That's a good point, I need to double check on that side. > > > >>> You explained it fine. Indeed SPF is handling deferred TLB invalidation by > >>> marking the VMA through vm_write_begin/end(), as for the fork case you > >>> mentioned. Once the PTL is held, and the VMA's seqcount is checked, the PTE > >>> values read are valid. > >> That should indeed work, but are we really sure we covered them all? > >> Should we invest in better TLBI APIs to make sure we can't get this > >> wrong? > > > > That may be a good option to identify deferred TLB invalidation but I've no clue on what this API would look like. > > I will send an RFC soon for per-table deferred TLB flushes tracking. > The basic idea is to save a generation in the page-struct that tracks > when deferred PTE change took place, and track whenever a TLB flush > completed. In addition, other users - such as mprotect - would use > the tlb_gather interface. > > Unfortunately, due to limited space in page-struct this would only > be possible for 64-bit (and my implementation is only for x86-64). I don't want to discourage you but I don't think this would end up well. PPC doesn't necessarily follow one-page-struct-per-table rule, and I've run into problems with this before while trying to do something similar. I'd recommend per-vma and per-category (unmapping, clearing writable and clearing dirty) tracking, which only rely on arch-independent data structures, i.e., vm_area_struct and mm_struct. > It would still require to do the copying while holding the PTL though. IMO, this is unacceptable. Most archs don't support per-table PTL, and even x86_64 can be configured to use per-mm PTL. What if we want to support a larger page size in the feature? It seems to me the only way to solve the problem with self-explanatory code and without performance impact is to check mm_tlb_flush_pending and the writable bit (and two other cases I mentioned above) at the same time. Of course, this requires a lot of effort to audit the existing uses, clean them up and properly wrap them up with new primitives, BUG_ON all invalid cases and document the exact workflow to prevent misuses. I've mentioned the following before -- it only demonstrates the rough idea. diff --git a/mm/memory.c b/mm/memory.c index 5e9ca612d7d7..af38c5ee327e 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -4403,8 +4403,11 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf) goto unlock; } if (vmf->flags & FAULT_FLAG_WRITE) { - if (!pte_write(entry)) + if (!pte_write(entry)) { + if (mm_tlb_flush_pending(vmf->vma->vm_mm)) + flush_tlb_page(vmf->vma, vmf->address); return do_wp_page(vmf); + } entry = pte_mkdirty(entry); } entry = pte_mkyoung(entry);
