Re: [PATCH] mm: don't call lru draining in the nested lru_cache_disable

[Andrew Morton <akpm@linux-foundation.org>]

On Mon,  6 Dec 2021 14:10:06 -0800 Minchan Kim <minchan@kernel.org> wrote:

> lru_cache_disable involves IPIs to drain pagevec of each core,
> which sometimes takes quite long time to complete depending
> on cpu's business, which makes allocation too slow up to
> sveral hundredth milliseconds. Furthermore, the repeated draining
> in the alloc_contig_range makes thing worse considering caller
> of alloc_contig_range usually tries multiple times in the loop.
> 
> This patch makes the lru_cache_disable aware of the fact the
> pagevec was already disabled. With that, user of alloc_contig_range
> can disable the lru cache in advance in their context during the
> repeated trial so they can avoid the multiple costly draining
> in cma allocation.

Isn't this racy?
 
> ...
>
> @@ -859,7 +869,12 @@ atomic_t lru_disable_count = ATOMIC_INIT(0);
>   */
>  void lru_cache_disable(void)
>  {
> -	atomic_inc(&lru_disable_count);
> +	/*
> +	 * If someone is already disabled lru_cache, just return with
> +	 * increasing the lru_disable_count.
> +	 */
> +	if (atomic_inc_not_zero(&lru_disable_count))
> +		return;
>  #ifdef CONFIG_SMP
>  	/*
>  	 * lru_add_drain_all in the force mode will schedule draining on
> @@ -873,6 +888,7 @@ void lru_cache_disable(void)
>  #else
>  	lru_add_and_bh_lrus_drain();
>  #endif

There's a window here where lru_disable_count==0 and new pages can get
added to lru?

> +	atomic_inc(&lru_disable_count);
>  }