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Date:   Sat, 27 May 2023 18:08:36 +0000
From:   Oliver Upton <oliver.upton@linux.dev>
To:     Yu Zhao <yuzhao@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
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Subject: Re: [PATCH mm-unstable v2 04/10] kvm/arm64: make stage2 page tables
 RCU safe
Message-ID: <ZHJHJPBF6euzOFdw@linux.dev>
References: <20230526234435.662652-1-yuzhao@google.com>
 <20230526234435.662652-5-yuzhao@google.com>
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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@google.com>
> ---
>  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@linux.dev>
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@linux.dev>

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);
 
 	return 0;
 }

-- 
Thanks,
Oliver