On Tue, Oct 25, 2022 at 04:18:20PM +0200, Jann Horn wrote: > On Tue, Oct 25, 2022 at 4:02 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > On Mon, Oct 24, 2022 at 09:58:07PM +0200, Jann Horn wrote: > > > > > Unless I'm completely misunderstanding what's going on here, the whole > > > "remove_table" thing only happens when you "remove a table", meaning > > > you free an entire *pagetable*. Just zapping PTEs doesn't trigger that > > > logic. > > > > Aah; yes true. OTOH even it that were not so, I think it would still be > > broken because the current code relies on the TLB flush to have > > completed, whereas the RCU scheme is effectively async and can be > > considered pending until the callback runs. > > > > Hurmph... easiest fix is probably to dis-allow kvm_flush_tlb_multi() > > for i386-pae builds. > > > > Something like so... nobody in his right mind should care about i386-pae > > virt performance much. > > I think Xen and HyperV have similar codepaths. > hyperv_flush_tlb_multi() looks like it uses remote flush hypercalls, > xen_flush_tlb_multi() too. Sure (not updated). > On top of that, I think that theoretically, Linux doesn't even ensure > that you have a TLB flush in between tearing down one PTE and > installing another PTE (see > https://lore.kernel.org/all/CAG48ez1Oz4tT-N2Y=Zs6jumu=zOp7SQRZ=V2c+b5bT9P4retJA@xxxxxxxxxxxxxx/), > but I haven't tested that, and if it is true, I'm also not entirely > sure if it's correct (in the sense that it only creates incoherent-TLB > states when userspace is doing something stupid like racing > MADV_DONTNEED and page faults on the same region). > > I think the more clearly correct fix would be to get rid of the split > loads and use CMPXCHG16B instead (probably destroying the performance > of GUP-fast completely), but that's complicated because some of the > architectures that use the split loads path don't have cmpxchg_double > (or at least don't have it wired up). cmpxchg8b; but no, I think we want to fix MADV_DONTNEED, incoherent TLB states are a pain nobody needs. Something like so should force TLB flushes before dropping pte_lock (not looked at the various pmd level things yet). diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c index 95fb85bea111..cbfb84e88251 100644 --- a/arch/x86/kernel/kvm.c +++ b/arch/x86/kernel/kvm.c @@ -473,6 +473,12 @@ static DEFINE_PER_CPU(cpumask_var_t, __pv_cpu_mask); static bool pv_tlb_flush_supported(void) { + /* + * i386-PAE split loads are incompatible with optimized TLB flushes. + */ + if (IS_ENABLED(CONFIG_GUP_GET_PTE_LOW_HIGH)) + return false; + return (kvm_para_has_feature(KVM_FEATURE_PV_TLB_FLUSH) && !kvm_para_has_hint(KVM_HINTS_REALTIME) && kvm_para_has_feature(KVM_FEATURE_STEAL_TIME) && diff --git a/include/linux/mm.h b/include/linux/mm.h index 8bbcccbc5565..397bc04e2d82 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -3474,5 +3474,6 @@ madvise_set_anon_name(struct mm_struct *mm, unsigned long start, * default, the flag is not set. */ #define ZAP_FLAG_DROP_MARKER ((__force zap_flags_t) BIT(0)) +#define ZAP_FLAG_FORCE_FLUSH ((__force zap_flags_t) BIT(1)) #endif /* _LINUX_MM_H */ diff --git a/mm/memory.c b/mm/memory.c index f88c351aecd4..9bb63b3fbee1 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -1440,6 +1440,11 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb, tlb_remove_tlb_entry(tlb, pte, addr); zap_install_uffd_wp_if_needed(vma, addr, pte, details, ptent); + + if (!force_flush && !tlb->fullmm && details && + details->zap_flags & ZAP_FLAG_FORCE_FLUSH) + force_flush = 1; + if (unlikely(!page)) continue; @@ -1749,6 +1754,9 @@ void zap_page_range(struct vm_area_struct *vma, unsigned long start, struct maple_tree *mt = &vma->vm_mm->mm_mt; unsigned long end = start + size; struct mmu_notifier_range range; + struct zap_details details = { + .zap_flags = ZAP_FLAG_FORCE_FLUSH, + }; struct mmu_gather tlb; MA_STATE(mas, mt, vma->vm_end, vma->vm_end); @@ -1759,7 +1767,7 @@ void zap_page_range(struct vm_area_struct *vma, unsigned long start, update_hiwater_rss(vma->vm_mm); mmu_notifier_invalidate_range_start(&range); do { - unmap_single_vma(&tlb, vma, start, range.end, NULL); + unmap_single_vma(&tlb, vma, start, range.end, &details); } while ((vma = mas_find(&mas, end - 1)) != NULL); mmu_notifier_invalidate_range_end(&range); tlb_finish_mmu(&tlb); @@ -1806,7 +1814,7 @@ void zap_vma_ptes(struct vm_area_struct *vma, unsigned long address, unsigned long size) { if (!range_in_vma(vma, address, address + size) || - !(vma->vm_flags & VM_PFNMAP)) + !(vma->vm_flags & VM_PFNMAP)) return; zap_page_range_single(vma, address, size, NULL);
