Yu,

On Fri, May 26, 2023 at 05:44:29PM -0600, Yu Zhao wrote:

quoted

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 <redacted>
---
 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 [off-list ref]
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 [off-list ref]

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);