On 24/11/25 6:52 PM, Kevin Brodsky wrote:
> The lazy MMU mode cannot be used in interrupt context. This is
> documented in <linux/pgtable.h>, but isn't consistently handled
> across architectures.
>
> arm64 ensures that calls to lazy_mmu_mode_* have no effect in
> interrupt context, because such calls do occur in certain
> configurations - see commit b81c688426a9 ("arm64/mm: Disable barrier
> batching in interrupt contexts"). Other architectures do not check
> this situation, most likely because it hasn't occurred so far.
>
> Let's handle this in the new generic lazy_mmu layer, in the same
> fashion as arm64: bail out of lazy_mmu_mode_* if in_interrupt().
> Also remove the arm64 handling that is now redundant.
Ensuring a common behaviour across platforms such as this in interrupt
context bail out - can now be easily achieved now as there are generic
helpers that call the platform ones.
> > Both arm64 and x86/Xen also ensure that any lazy MMU optimisation is
> disabled while in interrupt (see queue_pte_barriers() and
> xen_get_lazy_mode() respectively). This will be handled in the
> generic layer in a subsequent patch.
Makes sense.
>
> Signed-off-by: Kevin Brodsky <[email protected]>
> ---
> arch/arm64/include/asm/pgtable.h | 9 ---------
> include/linux/pgtable.h | 17 +++++++++++++++--
> 2 files changed, 15 insertions(+), 11 deletions(-)
>
> diff --git a/arch/arm64/include/asm/pgtable.h
> b/arch/arm64/include/asm/pgtable.h
> index 54f8d6bb6f22..e596899f4029 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -94,26 +94,17 @@ static inline void arch_enter_lazy_mmu_mode(void)
> * keeps tracking simple.
> */
>
> - if (in_interrupt())
> - return;
> -
> set_thread_flag(TIF_LAZY_MMU);
> }
>
> 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);
> }
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index c121358dba15..8ff6fdb4b13d 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -228,27 +228,40 @@ static inline int pmd_dirty(pmd_t pmd)
> * of the lazy mode. So the implementation must assume preemption may be
> enabled
> * and cpu migration is possible; it must take steps to be robust against
> this.
> * (In practice, for user PTE updates, the appropriate page table lock(s) are
> - * held, but for kernel PTE updates, no lock is held). Nesting is not
> permitted
> - * and the mode cannot be used in interrupt context.
> + * held, but for kernel PTE updates, no lock is held). The mode is disabled
> in
> + * interrupt context and calls to the lazy_mmu API have no effect.
> + * Nesting is not permitted.
Right - nesting is still not not permitted.
> */> #ifdef CONFIG_ARCH_HAS_LAZY_MMU_MODE
> static inline void lazy_mmu_mode_enable(void)
> {
> + if (in_interrupt())
> + return;
> +
> arch_enter_lazy_mmu_mode();
> }
>
> static inline void lazy_mmu_mode_disable(void)
> {
> + if (in_interrupt())
> + return;
> +
> arch_leave_lazy_mmu_mode();
> }
>
> static inline void lazy_mmu_mode_pause(void)
> {
> + if (in_interrupt())
> + return;
> +
> arch_leave_lazy_mmu_mode();
> }
>
> static inline void lazy_mmu_mode_resume(void)
> {
> + if (in_interrupt())
> + return;
> +
> arch_enter_lazy_mmu_mode();
> }
> #else
LGTM
Reviewed-by: Anshuman Khandual <[email protected]>
