linux-kernel - [PATCH v2 3/3] alloc_tag: avoid warnings when freeing non-compound "tail" pages

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Message-ID: <20250915212756.3998938-4-surenb@google.com>
Date: Mon, 15 Sep 2025 14:27:56 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: akpm@...ux-foundation.org
Cc: kent.overstreet@...ux.dev, vbabka@...e.cz, hannes@...xchg.org, 
	usamaarif642@...il.com, shakeel.butt@...ux.dev, 00107082@....com, 
	pasha.tatashin@...een.com, souravpanda@...gle.com, surenb@...gle.com, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: [PATCH v2 3/3] alloc_tag: avoid warnings when freeing non-compound
 "tail" pages

When freeing "tail" pages of a non-compount high-order page, we properly
subtract the allocation tag counters, however later when these pages are
released, alloc_tag_sub() will issue warnings because tags for these pages
are NULL.
This issue was originally anticipated by Vlastimil in his review [1] and
then recently reported by David.
Prevent warnings by marking the tags empty.

[1] https://lore.kernel.org/all/6db0f0c8-81cb-4d04-9560-ba73d63db4b8@suse.cz/

Suggested-by: David Wang <00107082@....com>
Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
Acked-by: Shakeel Butt <shakeel.butt@...ux.dev>
---
 mm/page_alloc.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index f54c5ee1e318..2bfab96c207f 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5240,9 +5240,16 @@ static void ___free_pages(struct page *page, unsigned int order,
 		__free_frozen_pages(page, order, fpi_flags);
 	else if (!head) {
 		pgalloc_tag_sub_pages(tag, (1 << order) - 1);
-		while (order-- > 0)
+		while (order-- > 0) {
+			/*
+			 * The "tail" pages of this non-compound high-order
+			 * page will have no code tags, so to avoid warnings
+			 * mark them as empty.
+			 */
+			clear_page_tag_ref(page + (1 << order));
 			__free_frozen_pages(page + (1 << order), order,
 					    fpi_flags);
+		}
 	}
 }
 
-- 
2.51.0.384.g4c02a37b29-goog