linux-kernel - [PATCH] mm/page_alloc: Warn on nr_reserved

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <20250225-warn-underflow-v1-1-3dc542941d3a@google.com>
Date: Tue, 25 Feb 2025 18:45:09 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Johannes Weiner <hannes@...xchg.org>, Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Brendan Jackman <jackmanb@...gle.com>
Subject: [PATCH] mm/page_alloc: Warn on nr_reserved_highatomic underflow

As documented in the comment this underflow should not happen. The
locking has indeed changed here since the comment was written, see the
migratetype hygiene patches[0]. However, those changes made the locking
_safer_, so the underflow _really_ shouldn't happen now. So upgrade
the comment to a warning.

[0] https://lore.kernel.org/all/20240320180429.678181-7-hannes@cmpxchg.org/T/#m3da87e6cc3348a4640aa298137bc9f8f61b76c84

Signed-off-by: Brendan Jackman <jackmanb@...gle.com>
---
 mm/page_alloc.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 5d8e274c8b1d500d263a17ef36fe190f60b88196..715a9cfe162090cca9eb819a34c64f9a1c6db29a 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3095,6 +3095,7 @@ static bool unreserve_highatomic_pageblock(const struct alloc_context *ac,
 			if (!page)
 				continue;
 
+			size = max(pageblock_nr_pages, 1UL << order);
 			/*
 			 * It should never happen but changes to
 			 * locking could inadvertently allow a per-cpu
@@ -3102,8 +3103,8 @@ static bool unreserve_highatomic_pageblock(const struct alloc_context *ac,
 			 * while unreserving so be safe and watch for
 			 * underflows.
 			 */
-			size = max(pageblock_nr_pages, 1UL << order);
-			size = min(size, zone->nr_reserved_highatomic);
+			if (WARN_ON_ONCE(size > zone->nr_reserved_highatomic))
+				size = zone->nr_reserved_highatomic;
 			zone->nr_reserved_highatomic -= size;
 
 			/*

---
base-commit: 0c789105c9d6c65777c995f4935f2e119d5a31a5
change-id: 20250225-warn-underflow-7ab0069182e2

Best regards,
-- 
Brendan Jackman <jackmanb@...gle.com>