Re: [PATCH] arm64/mm: Disable barrier batching in interrupt contexts

[David Hildenbrand <david@redhat.com>]

On 12.05.25 12:22, Ryan Roberts wrote:
> Commit 5fdd05efa1cd ("arm64/mm: Batch barriers when updating kernel
> mappings") enabled arm64 kernels to track "lazy mmu mode" using TIF
> flags in order to defer barriers until exiting the mode. At the same
> time, it added warnings to check that pte manipulations were never
> performed in interrupt context, because the tracking implementation
> could not deal with nesting.
> 
> But it turns out that some debug features (e.g. KFENCE, DEBUG_PAGEALLOC)
> do manipulate ptes in softirq context, which triggered the warnings.
> 
> So let's take the simplest and safest route and disable the batching
> optimization in interrupt contexts. This makes these users no worse off
> than prior to the optimization. Additionally the known offenders are
> debug features that only manipulate a single PTE, so there is no
> performance gain anyway.
> 
> There may be some obscure case of encrypted/decrypted DMA with the
> dma_free_coherent called from an interrupt context, but again, this is
> no worse off than prior to the commit.
> 
> Some options for supporting nesting were considered, but there is a
> difficult to solve problem if any code manipulates ptes within interrupt
> context but *outside of* a lazy mmu region. If this case exists, the
> code would expect the updates to be immediate, but because the task
> context may have already been in lazy mmu mode, the updates would be
> deferred, which could cause incorrect behaviour. This problem is avoided
> by always ensuring updates within interrupt context are immediate.
> 
> Fixes: 5fdd05efa1cd ("arm64/mm: Batch barriers when updating kernel mappings")
> Reported-by: syzbot+5c0d9392e042f41d45c5@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/linux-arm-kernel/681f2a09.050a0220.f2294.0006.GAE@google.com/
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
> 
> Hi Will,
> 
> I've tested before and after with KFENCE enabled and it solves the issue. I've
> also run all the mm-selftests which all continue to pass.
> 
> Catalin suggested a Fixes patch targetting the SHA as it is in for-next/mm was
> the preferred approach, but shout if you want something different. I'm hoping
> that with this fix we can still make it for this cycle, subject to not finding
> any more issues.
> 
> Thanks,
> Ryan
> 
> 
>   arch/arm64/include/asm/pgtable.h | 16 ++++++++++++++--
>   1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index ab4a1b19e596..e65083ec35cb 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -64,7 +64,11 @@ static inline void queue_pte_barriers(void)
>   {
>   	unsigned long flags;
> 
> -	VM_WARN_ON(in_interrupt());
> +	if (in_interrupt()) {
> +		emit_pte_barriers();
> +		return;
> +	}
> +
>   	flags = read_thread_flags();
> 
>   	if (flags & BIT(TIF_LAZY_MMU)) {
> @@ -79,7 +83,9 @@ static inline void queue_pte_barriers(void)
>   #define  __HAVE_ARCH_ENTER_LAZY_MMU_MODE
>   static inline void arch_enter_lazy_mmu_mode(void)
>   {
> -	VM_WARN_ON(in_interrupt());
> +	if (in_interrupt())
> +		return;
> +
>   	VM_WARN_ON(test_thread_flag(TIF_LAZY_MMU));
> 
>   	set_thread_flag(TIF_LAZY_MMU);
> @@ -87,12 +93,18 @@ static inline void arch_enter_lazy_mmu_mode(void)
> 
>   static inline void arch_flush_lazy_mmu_mode(void)
>   {
> +	if (in_interrupt())
> +		return;
> +
>   	if (test_and_clear_thread_flag(TIF_LAZY_MMU_PENDING))
>   		emit_pte_barriers();
>   }
> 
>   static inline void arch_leave_lazy_mmu_mode(void)
>   {
> +	if (in_interrupt())
> +		return;
> +
>   	arch_flush_lazy_mmu_mode();
>   	clear_thread_flag(TIF_LAZY_MMU);
>   }

I guess in all cases we could optimize out the in_interrupt() check on 
!debug configs.

Hm, maybe there is an elegant way to catch all of these "problematic" users?

-- 
Cheers,

David / dhildenb