Re: [PATCH v2 2/2] mm: migrate: fix barriers around tlb_flush_pending

[Minchan Kim <minchan@kernel.org>]

On Wed, Jul 26, 2017 at 08:02:14AM -0700, Nadav Amit wrote:
> Reading tlb_flush_pending while the page-table lock is taken does not
> require a barrier, since the lock/unlock already acts as a barrier.
> Removing the barrier in mm_tlb_flush_pending() to address this issue.
> 
> However, migrate_misplaced_transhuge_page() calls mm_tlb_flush_pending()
> while the page-table lock is already released, which may present a
> problem on architectures with weak memory model (PPC). Use
> smp_mb__after_unlock_lock() in that case.
> 
> Signed-off-by: Nadav Amit <namit@vmware.com>
> ---
>  include/linux/mm_types.h | 18 ++++++++++++------
>  mm/migrate.c             |  9 +++++++++
>  2 files changed, 21 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 36f4ec589544..312eec5690d4 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -522,12 +522,12 @@ static inline cpumask_t *mm_cpumask(struct mm_struct *mm)
>  /*
>   * Memory barriers to keep this state in sync are graciously provided by
>   * the page table locks, outside of which no page table modifications happen.
> - * The barriers below prevent the compiler from re-ordering the instructions
> - * around the memory barriers that are already present in the code.
> + * The barriers are used to ensure the order between tlb_flush_pending updates,
> + * which happen while the lock is not taken, and the PTE updates, which happen
> + * while the lock is taken, are serialized.
>   */
>  static inline bool mm_tlb_flush_pending(struct mm_struct *mm)
>  {
> -	barrier();
>  	return atomic_read(&mm->tlb_flush_pending) > 0;
>  }
>  static inline void set_tlb_flush_pending(struct mm_struct *mm)
> @@ -535,15 +535,21 @@ static inline void set_tlb_flush_pending(struct mm_struct *mm)
>  	atomic_inc(&mm->tlb_flush_pending);
>  
>  	/*
> -	 * Guarantee that the tlb_flush_pending store does not leak into the
> +	 * Guarantee that the tlb_flush_pending increase does not leak into the
>  	 * critical section updating the page tables
>  	 */
>  	smp_mb__before_spinlock();
>  }
> -/* Clearing is done after a TLB flush, which also provides a barrier. */
> +
>  static inline void clear_tlb_flush_pending(struct mm_struct *mm)
>  {
> -	barrier();
> +	/*
> +	 * Guarantee that the tlb_flush_pending does not not leak into the
> +	 * critical section, since we must order the PTE change and changes to
> +	 * the pending TLB flush indication. We could have relied on TLB flush
> +	 * as a memory barrier, but this behavior is not clearly documented.
> +	 */
> +	smp_mb__before_atomic();
>  	atomic_dec(&mm->tlb_flush_pending);
>  }
>  #else
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 89a0a1707f4c..85c7134d70cc 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1935,6 +1935,15 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
>  		put_page(new_page);
>  		goto out_fail;
>  	}
> +
> +	/*
> +	 * mm_tlb_flush_pending() is safe if it is executed while the page-table
> +	 * lock is taken. But here, it is executed while the page-table lock is
> +	 * already released. This requires a full memory barrier on
> +	 * architectures with weak memory models.
> +	 */
> +	smp_mb__after_unlock_lock();
> +

As you saw my work, I will use mm_tlb_flush_pending in tlb_finish_mmu where
page-table lock is already released. So, I should use same comment/barrier
in there, too.

Like that, mm_tlb_flush_pending user should be aware of whether he is
calling the mm_tlb_flush_pending inside of pte lock or not.
I think it would be better to say about it as function interface.
IOW,

        bool mm_tlb_flush_pending(bool pte_locked)

Otherwise, at least, I hope comment you wrote in here should be in
mm_tlb_flush_pending for users to catch it up.

Thanks.

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