linux-kernel - Re: [PATCH] mm:vmscan: fix shrink sc->nr counter values issue

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Date:   Wed, 29 Nov 2023 15:17:43 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Zhiguo Jiang <justinjiang@...o.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, opensource.kernel@...o.com
Subject: Re: [PATCH] mm:vmscan: fix shrink sc->nr counter values issue

On Wed, Nov 29, 2023 at 09:01:26PM +0800, Zhiguo Jiang wrote:
> It is needed to ensure sc->nr.unqueued_dirty > 0, which can avoid to
> set PGDAT_DIRTY flag when sc->nr.unqueued_dirty and sc->nr.file_taken
> are both zero at the same time.

Have you observed this happening, or is this from code review?

> It can't be guaranteed for the PGDAT_WRITEBACK flag that only pages
> marked for immediate reclaim are on evictable LRUs in other following
> shrink processes of the same kswapd shrink recycling. So when both a
> small amount of pages marked for immediate reclaim and a large amount
> of pages marked for non-immediate reclaim are on evictable LRUs at the
> same time, if it's only determined that there is at least a page marked
> for immediate reclaim on evictable LRUs, kswapd shrink is throttled to
> sleep, which will increase kswapd process consumption.
> 
> It can be fixed to throttle kswapd shrink when sc->nr.immediate is equal
> to sc->nr.file_taken.

So you're fixing two distinct things in the same patch?

> +++ b/mm/vmscan.c
> @@ -5915,17 +5915,17 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>  			set_bit(PGDAT_WRITEBACK, &pgdat->flags);
>  
>  		/* Allow kswapd to start writing pages during reclaim.*/
> -		if (sc->nr.unqueued_dirty == sc->nr.file_taken)
> +		if (sc->nr.unqueued_dirty && sc->nr.unqueued_dirty == sc->nr.file_taken)
>  			set_bit(PGDAT_DIRTY, &pgdat->flags);
>  
>  		/*
> -		 * If kswapd scans pages marked for immediate
> +		 * If kswapd scans massive pages marked for immediate

I don't understand why you've added the word "massive".  Do you mean
that the pages are large, or that kswapd has scanned a lot of pages?

>  		 * reclaim and under writeback (nr_immediate), it
>  		 * implies that pages are cycling through the LRU
>  		 * faster than they are written so forcibly stall
>  		 * until some pages complete writeback.
>  		 */
> -		if (sc->nr.immediate)
> +		if (sc->nr.immediate && sc->nr.immediate == sc->nr.file_taken)
>  			reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK);
>  	}