Re: [PATCH v2] mm/page_alloc: don't wake kswapd from rmqueue() unless __GFP_KSWAPD_RECLAIM is specified

["Huang, Ying" <ying.huang@intel.com>]

Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> writes:

> 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.
>
> Only allocation contexts that include ALLOC_KSWAPD (which corresponds to
> __GFP_KSWAPD_RECLAIM) should wake kswapd, for callers are supposed to
> remove __GFP_KSWAPD_RECLAIM if trying to hold pgdat->kswapd_wait has a
> risk of deadlock. 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, causing possibility of deadlock.

Sorry, I don't understand what is the deadlock here.

I checked commit 73444bc4d8f9 ("mm, page_alloc: do not wake kswapd with
zone lock held") and the corresponding mail thread.  From the below
mail,

https://lore.kernel.org/all/20190107204627.GA25526@cmpxchg.org/

commit 73444bc4d8f9 fixed a circular locking ordering as follows,

pi lock -> rq lock -> timer base lock -> zone lock -> wakeup lock
(kswapd_wait, fixed) -> pi lock

But I don't know what is the deadlock that your patch fixed.  Can you
teach me on that?

Best Regards,
Huang, Ying

> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Fixes: 73444bc4d8f9 ("mm, page_alloc: do not wake kswapd with zone lock held")
> ---
> Changes in v2:
>   Check ALLOC_KSWAPD before checking ZONE_BOOSTED_WATERMARK and update
>   description, suggested by Mel Gorman <mgorman@techsingularity.net>.
>
>  mm/page_alloc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 47421bedc12b..ecad680cec53 100644
> --- a/mm/page_alloc.c
> +++ 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 ((alloc_flags & ALLOC_KSWAPD) &&
> +	    unlikely(test_bit(ZONE_BOOSTED_WATERMARK, &zone->flags))) {
>  		clear_bit(ZONE_BOOSTED_WATERMARK, &zone->flags);
>  		wakeup_kswapd(zone, 0, 0, zone_idx(zone));
>  	}