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

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

On 2023/05/12 12:45, Andrew Morton wrote:
> 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?

Potential deadlock upon __GFP_HIGH (I mean, !__GFP_KSWAPD_RECLAIM)
allocation requests (like debugobject code is about to start doing).

> 
>> 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.
>