Re: [PATCH] mm/page_alloc: don't wake up kswapd from rmqueue() unless __GFP_KSWAPD_RECLAIM is specified

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

On Thu, 11 May 2023 22:47:36 +0900 Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:

> Commit 73444bc4d8f9 ("mm, page_alloc: do not wake kswapd with zone lock
> held") moved wakeup_kswapd() from steal_suitable_fallback() to rmqueue()
> using ZONE_BOOSTED_WATERMARK flag. But since zone->flags is a shared
> variable, a thread doing !__GFP_KSWAPD_RECLAIM allocation request might
> observe this flag being set immediately after another thread doing
> __GFP_KSWAPD_RECLAIM allocation request set this flag.

What are the user-visible runtime effects of this flaw?

> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.j
> +++ b/mm/page_alloc.c
> @@ -3052,7 +3052,8 @@ struct page *rmqueue(struct zone *preferred_zone,
>  
>  out:
>  	/* Separate test+clear to avoid unnecessary atomics */
> -	if (unlikely(test_bit(ZONE_BOOSTED_WATERMARK, &zone->flags))) {
> +	if (unlikely(test_bit(ZONE_BOOSTED_WATERMARK, &zone->flags))
> +	    && (alloc_flags & ALLOC_KSWAPD)) {
>  		clear_bit(ZONE_BOOSTED_WATERMARK, &zone->flags);
>  		wakeup_kswapd(zone, 0, 0, zone_idx(zone));
>  	}

Thanks, I'll queue this up for some testing while awaiting input from
Mel.