linux-kernel - Re: [PATCH 1/6] mm: page_alloc: remove pcppage migratetype caching

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Message-ID: <20230912150320.GB3228@cmpxchg.org>
Date:   Tue, 12 Sep 2023 11:03:20 -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>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Zi Yan <ziy@...dia.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] mm: page_alloc: remove pcppage migratetype caching

On Tue, Sep 12, 2023 at 03:47:45PM +0200, Vlastimil Babka wrote:
> I think after this change we should [...]

Speaking of follow-ups, AFAICS we no longer need those either:

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 9cad31de1bf5..bea499fbca58 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1751,13 +1751,6 @@ static void steal_suitable_fallback(struct zone *zone, struct page *page,
 
 	old_block_type = get_pageblock_migratetype(page);
 
-	/*
-	 * This can happen due to races and we want to prevent broken
-	 * highatomic accounting.
-	 */
-	if (is_migrate_highatomic(old_block_type))
-		goto single_page;
-
 	/* Take ownership for orders >= pageblock_order */
 	if (current_order >= pageblock_order) {
 		change_pageblock_range(page, current_order, start_type);
@@ -1926,24 +1919,15 @@ static bool unreserve_highatomic_pageblock(const struct alloc_context *ac,
 				continue;
 
 			/*
-			 * In page freeing path, migratetype change is racy so
-			 * we can counter several free pages in a pageblock
-			 * in this loop although we changed the pageblock type
-			 * from highatomic to ac->migratetype. So we should
-			 * adjust the count once.
+			 * It should never happen but changes to
+			 * locking could inadvertently allow a per-cpu
+			 * drain to add pages to MIGRATE_HIGHATOMIC
+			 * while unreserving so be safe and watch for
+			 * underflows.
 			 */
-			if (is_migrate_highatomic_page(page)) {
-				/*
-				 * It should never happen but changes to
-				 * locking could inadvertently allow a per-cpu
-				 * drain to add pages to MIGRATE_HIGHATOMIC
-				 * while unreserving so be safe and watch for
-				 * underflows.
-				 */
-				zone->nr_reserved_highatomic -= min(
-						pageblock_nr_pages,
-						zone->nr_reserved_highatomic);
-			}
+			zone->nr_reserved_highatomic -= min(
+				pageblock_nr_pages,
+				zone->nr_reserved_highatomic);
 
 			/*
 			 * Convert to ac->migratetype and avoid the normal

I think they were only in place because we could change the highatomic
status of pages on the pcplist, and those pages would then end up on
some other freelist due to the stale pcppage cache.

I replaced them locally with WARNs and ran an hour or so of kernel
builds under pressure. It didn't trigger. So I would send a follow up
to remove them.

Unless you point me to a good reason why they're definitely still
needed - in which case this is a moot proposal - but then we should
make the comments more specific.