On Thu, Jan 27, 2022 at 07:24:37PM +0000, Valentin Schneider wrote: > arch_faults_on_old_pte() relies on the calling context being > non-preemptible. CONFIG_PREEMPT_RT turns the PTE lock into a sleepable > spinlock, which doesn't disable preemption once acquired, triggering the > warning in arch_faults_on_old_pte(). > > It does however disable migration, ensuring the task remains on the same > CPU during the entirety of the critical section, making the read of > cpu_has_hw_af() safe and stable. > > Make arch_faults_on_old_pte() check cant_migrate() instead of preemptible(). > > Suggested-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> > Signed-off-by: Valentin Schneider <valentin.schneider@xxxxxxx> > Link: https://lore.kernel.org/r/20210811201354.1976839-5-valentin.schneider@xxxxxxx > --- > v3 -> v4: Dropped migratable(), reuse cant_migrate() (Sebastian) > --- > arch/arm64/include/asm/pgtable.h | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h > index c4ba047a82d2..3caf6346ea95 100644 > --- a/arch/arm64/include/asm/pgtable.h > +++ b/arch/arm64/include/asm/pgtable.h > @@ -1001,7 +1001,8 @@ static inline void update_mmu_cache(struct vm_area_struct *vma, > */ > static inline bool arch_faults_on_old_pte(void) > { > - WARN_ON(preemptible()); > + /* The register read below requires a stable CPU to make any sense */ > + cant_migrate(); FWIW: There's a patch in the multi-generational LRU series that drops this WARN_ON() entirely. I'm not sure if/when that lot will land, but just wanted to let you know. Will
