linux-kernel - Re: [PATCH 1/5] mm: compaction: push watermark into compaction

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Message-ID: <20250317181851.GA1694060@cmpxchg.org>
Date: Mon, 17 Mar 2025 14:18:51 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Hugh Dickins <hughd@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...pxchg.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	Mel Gorman <mgorman@...hsingularity.net>, Zi Yan <ziy@...dia.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] mm: compaction: push watermark into
 compaction_suitable() callers

On Sat, Mar 15, 2025 at 09:28:16PM -0700, Hugh Dickins wrote:
> [PATCH] mm: compaction: push watermark into compaction_suitable() callers fix
> 
> Stop oops on out-of-range highest_zoneidx: compaction_suitable() pass
> args to __compaction_suitable() in the order which it is expecting.
> 
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> ---
>  mm/compaction.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 4a2ccb82d0b2..b5c9e8fd39f1 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -2433,7 +2433,7 @@ bool compaction_suitable(struct zone *zone, int order, unsigned long watermark,
>  	enum compact_result compact_result;
>  	bool suitable;
>  
> -	suitable = __compaction_suitable(zone, order, highest_zoneidx, watermark,
> +	suitable = __compaction_suitable(zone, order, watermark, highest_zoneidx,
>  					 zone_page_state(zone, NR_FREE_PAGES));

Ouch, thanks for the fix Hugh.

This obviously didn't crash for me, but I re-ran the benchmarks with
your fix in my test environment.

This affects the direct compaction path, and I indeed see a minor
uptick in direct compaction, with a larger reduction in daemon work.

Compact daemon scanned migrate        2455570.93 (    +0.00%)   1770142.33 (   -27.91%)
Compact daemon scanned free           2429309.20 (    +0.00%)   1604744.00 (   -33.94%)
Compact direct scanned migrate          40136.60 (    +0.00%)     58326.67 (   +45.32%)
Compact direct scanned free             22127.13 (    +0.00%)     52216.93 (  +135.98%)
Compact total migrate scanned         2495707.53 (    +0.00%)   1828469.00 (   -26.74%)
Compact total free scanned            2451436.33 (    +0.00%)   1656960.93 (   -32.41%)

It doesn't change the overall A/B picture between baseline and the
series, so I'm comfortable keeping the current changelog results.