FAILED: patch "[PATCH] mm/page_table_check: skip zero pages" failed to apply to 6.12-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x 33afd07412a1efe5b750a3fd5eb0272ebdd12761
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107051936-failed-6.12-33afd07412a1@kernel.org' --subject-prefix 'PATCH 6.12.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 33afd07412a1efe5b750a3fd5eb0272ebdd12761 Mon Sep 17 00:00:00 2001
From: Zhiling Zou <roxy520tt@gmail.com>
Date: Tue, 21 Jul 2026 23:56:38 +0800
Subject: [PATCH] mm/page_table_check: skip zero pages

page_table_check_set() accounts pages by whether they are PageAnon().  The
shared zero page is a special page, not an ordinary file-backed page.
Read faults on private anonymous mappings can install many read-only PTEs
that point at the zero page, but page_table_check currently accounts them
in file_map_count.

That lets an unprivileged process populate enough zero-page mappings to
overflow file_map_count and trip the BUG_ON() in page_table_check_set().

Skip zero pages in page_table_check accounting.  They do not need the
anonymous/file mapping conflict checks that page_table_check performs for
ordinary pages, and this keeps the existing counter size and page_ext
layout unchanged.

Link: https://lore.kernel.org/1f8848512d2e3ded944f8d595c29faee8fdaeab0.1784645969.git.roxy520tt@gmail.com
Fixes: df4e817b7108 ("mm: page table check")
Signed-off-by: Zhiling Zou <roxy520tt@gmail.com>
Signed-off-by: Ren Wei <enjou1224z@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Vega <vega@nebusec.ai>
Assisted-by: Codex:gpt-5.4
Cc: Ye Liu <liuye@kylinos.cn>
Cc: <stable@vger.kernel.org>
---
 mm/page_table_check.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/page_table_check.c b/mm/page_table_check.c
index ed3e1a76f2664..143a918c0bde2 100644
--- a/mm/page_table_check.c
+++ b/mm/page_table_check.c
@@ -67,7 +67,7 @@ static void page_table_check_clear(unsigned long pfn, unsigned long pgcnt)
 	struct page *page;
 	bool anon;
 
-	if (!pfn_valid(pfn))
+	if (!pfn_valid(pfn) || is_zero_pfn(pfn) || is_huge_zero_pfn(pfn))
 		return;
 
 	page = pfn_to_page(pfn);
@@ -102,7 +102,7 @@ static void page_table_check_set(unsigned long pfn, unsigned long pgcnt,
 	struct page *page;
 	bool anon;
 
-	if (!pfn_valid(pfn))
+	if (!pfn_valid(pfn) || is_zero_pfn(pfn) || is_huge_zero_pfn(pfn))
 		return;
 
 	page = pfn_to_page(pfn);
-- 
2.53.0