linux-kernel - Re: [PATCH 2/5] mm: compaction: simplify should_compact

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Date:   Fri, 2 Jun 2023 10:47:05 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...e.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH 2/5] mm: compaction: simplify should_compact_retry()

On Mon, May 29, 2023 at 12:38:07PM -0400, Johannes Weiner wrote:
> On Mon, May 29, 2023 at 03:03:52PM +0200, Vlastimil Babka wrote:
> > I think you simplified this part too much, so now once it runs out of
> > retries, it will return false, while previously it would increase the priority.

Here is the delta fix. If this looks good to everybody, can you please
fold this into the patch you have in tree? Thanks!

---
>From 4b9429f9ef04fcb7bb5ffae0db8ea113b26d097b Mon Sep 17 00:00:00 2001
From: Johannes Weiner <hannes@...xchg.org>
Date: Fri, 2 Jun 2023 16:02:37 +0200
Subject: [PATCH] mm: compaction: simplify should_compact_retry() fix

Vlastimil points out an unintended change. Previously when hitting
max_retries we'd bump the priority level and restart the loop. Now we
bail out and fail instead. Restore the original behavior.

Signed-off-by: Johannes Weiner <hannes@...xchg.org>
---
 mm/page_alloc.c | 24 +++++++++++++-----------
 1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 72660e924b95..e7d7db36582b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3768,6 +3768,15 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
 	if (fatal_signal_pending(current))
 		return false;
 
+	/*
+	 * Compaction was skipped due to a lack of free order-0
+	 * migration targets. Continue if reclaim can help.
+	 */
+	if (compact_result == COMPACT_SKIPPED) {
+		ret = compaction_zonelist_suitable(ac, order, alloc_flags);
+		goto out;
+	}
+
 	/*
 	 * Compaction managed to coalesce some page blocks, but the
 	 * allocation failed presumably due to a race. Retry some.
@@ -3785,17 +3794,10 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
 		if (order > PAGE_ALLOC_COSTLY_ORDER)
 			max_retries /= 4;
 
-		ret = ++(*compaction_retries) <= max_retries;
-		goto out;
-	}
-
-	/*
-	 * Compaction was skipped due to a lack of free order-0
-	 * migration targets. Continue if reclaim can help.
-	 */
-	if (compact_result == COMPACT_SKIPPED) {
-		ret = compaction_zonelist_suitable(ac, order, alloc_flags);
-		goto out;
+		if (++(*compaction_retries) <= max_retries) {
+			ret = true;
+			goto out;
+		}
 	}
 
 	/*
-- 
2.40.1