Re: [PATCH v2] [RFC] mm: fs: Invalidate BH LRU during page migration

[Matthew Wilcox]

On Wed, Feb 10, 2021 at 09:35:40PM -0800, Chris Goldsworthy wrote:
> +/* These are used to control the BH LRU invalidation during page migration */
> +static struct cpumask lru_needs_invalidation;
> +static bool bh_lru_disabled = false;

As I asked before, what protects this on an SMP system?

> @@ -1292,7 +1296,9 @@ static inline void check_irqs_on(void)
>  /*
>   * Install a buffer_head into this cpu's LRU.  If not already in the LRU, it is
>   * inserted at the front, and the buffer_head at the back if any is evicted.
> - * Or, if already in the LRU it is moved to the front.
> + * Or, if already in the LRU it is moved to the front. Note that if LRU is
> + * disabled because of an ongoing page migration, we won't insert bh into the
> + * LRU.

And also, why do we need to do this?  The page LRU has no equivalent
mechanism to prevent new pages being added to the per-CPU LRU lists.
If a BH has just been used, isn't that a strong hint that this page is
a bad candidate for migration?