On Sat, May 27, 2023 at 12:08 PM Oliver Upton <oliver.upton@xxxxxxxxx> wrote: > > Yu, > > On Fri, May 26, 2023 at 05:44:29PM -0600, Yu Zhao wrote: > > Stage2 page tables are currently not RCU safe against unmapping or VM > > destruction. The previous mmu_notifier_ops members rely on > > kvm->mmu_lock to synchronize with those operations. > > > > However, the new mmu_notifier_ops member test_clear_young() provides > > a fast path that does not take kvm->mmu_lock. To implement > > kvm_arch_test_clear_young() for that path, unmapped page tables need > > to be freed by RCU and kvm_free_stage2_pgd() needs to be after > > mmu_notifier_unregister(). > > > > Remapping, specifically stage2_free_removed_table(), is already RCU > > safe. > > > > Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx> > > --- > > arch/arm64/include/asm/kvm_pgtable.h | 2 ++ > > arch/arm64/kvm/arm.c | 1 + > > arch/arm64/kvm/hyp/pgtable.c | 8 ++++++-- > > arch/arm64/kvm/mmu.c | 17 ++++++++++++++++- > > 4 files changed, 25 insertions(+), 3 deletions(-) > > > > diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h > > index ff520598b62c..5cab52e3a35f 100644 > > --- a/arch/arm64/include/asm/kvm_pgtable.h > > +++ b/arch/arm64/include/asm/kvm_pgtable.h > > @@ -153,6 +153,7 @@ static inline bool kvm_level_supports_block_mapping(u32 level) > > * @put_page: Decrement the refcount on a page. When the > > * refcount reaches 0 the page is automatically > > * freed. > > + * @put_page_rcu: RCU variant of the above. > > You don't need to add yet another hook to implement this. I was working > on lock-free walks in a separate context and arrived at the following: > > commit f82d264a37745e07ee28e116c336f139f681fd7f > Author: Oliver Upton <oliver.upton@xxxxxxxxx> > Date: Mon May 1 08:53:37 2023 +0000 > > KVM: arm64: Consistently use free_removed_table() for stage-2 > > free_removed_table() is essential to the RCU-protected parallel walking > scheme, as behind the scenes the cleanup is deferred until an RCU grace > period. Nonetheless, the stage-2 unmap path calls put_page() directly, > which leads to table memory being freed inline with the table walk. > > This is safe for the time being, as the stage-2 unmap walker is called > while holding the write lock. A future change to KVM will further relax > the locking mechanics around the stage-2 page tables to allow lock-free > walkers protected only by RCU. As such, switch to the RCU-safe mechanism > for freeing table memory. > > Signed-off-by: Oliver Upton <oliver.upton@xxxxxxxxx> > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c > index 3d61bd3e591d..bfbebdcb4ef0 100644 > --- a/arch/arm64/kvm/hyp/pgtable.c > +++ b/arch/arm64/kvm/hyp/pgtable.c > @@ -1019,7 +1019,7 @@ static int stage2_unmap_walker(const struct kvm_pgtable_visit_ctx *ctx, > kvm_granule_size(ctx->level)); > > if (childp) > - mm_ops->put_page(childp); > + mm_ops->free_removed_table(childp, ctx->level); Thanks, Oliver. A couple of things I haven't had the chance to verify -- I'm hoping you could help clarify: 1. For unmapping, with free_removed_table(), wouldn't we have to look into the table we know it's empty unnecessarily? 2. For remapping and unmapping, how does free_removed_table() put the final refcnt on the table passed in? (Previously we had put_page(childp) in stage2_map_walk_table_post(). So I'm assuming we'd have to do something equivalent with free_removed_table().)
