Re: [PATCH v1 4/4] mm: Allow detection of wrong arch_enter_lazy_mmu_mode() context

["Nicholas Piggin" <npiggin@gmail.com>]

On Tue Apr 8, 2025 at 1:11 AM AEST, Alexander Gordeev wrote:
> The lazy MMU batching may be only be entered and left under the
> protection of the page table locks for all page tables which may
> be modified. Yet, there were cases arch_enter_lazy_mmu_mode()
> was called without the locks taken, e.g. commit b9ef323ea168
> ("powerpc/64s: Disable preemption in hash lazy mmu mode").
>
> Make default arch_enter|leave|flush_lazy_mmu_mode() callbacks
> complain at least in case the preemption is enabled to detect
> wrong contexts.
>
> Most platforms do not implement the callbacks, so to aovid a
> performance impact allow the complaint when CONFIG_DEBUG_VM
> option is enabled only.
>
> Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

This is a good debugging feature independent of how the fix
is done. I would just warn once, since it's not a bug for
the arch and could fire frequently if it fires at all.

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>

> ---
>  include/linux/pgtable.h | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index e2b705c14945..959590bb66da 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -232,9 +232,18 @@ static inline int pmd_dirty(pmd_t pmd)
>   * and the mode cannot be used in interrupt context.
>   */
>  #ifndef __HAVE_ARCH_ENTER_LAZY_MMU_MODE
> -#define arch_enter_lazy_mmu_mode()	do {} while (0)
> -#define arch_leave_lazy_mmu_mode()	do {} while (0)
> -#define arch_flush_lazy_mmu_mode()	do {} while (0)
> +static inline void arch_enter_lazy_mmu_mode(void)
> +{
> +	VM_WARN_ON(preemptible());
> +}
> +static inline void arch_leave_lazy_mmu_mode(void)
> +{
> +	VM_WARN_ON(preemptible());
> +}
> +static inline void arch_flush_lazy_mmu_mode(void)
> +{
> +	VM_WARN_ON(preemptible());
> +}
>  #endif
>  
>  #ifndef pte_batch_hint