FAILED: patch "[PATCH] mm/mremap: fix moving PUD leaves" 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 440c8779757f09f290770b1d361dc1516a9d0b3e
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261109200915-failed-6.12-440c8779757f@kernel.org' --subject-prefix 'PATCH 6.12.y' 'HEAD^..'

Thanks,
Sasha

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

From 440c8779757f09f290770b1d361dc1516a9d0b3e Mon Sep 17 00:00:00 2001
From: Josef Bacik <josef@toxicpanda.com>
Date: Sat, 31 Oct 2026 10:40:01 +0100
Subject: [PATCH] mm/mremap: fix moving PUD leaves

pud_none_or_clear_bad() only really works on page tables. PUD leaves
confuse the check and make us clear the page table entry and print a
warning.

Since commit aed877c2b4257 ("device/dax: properly refcount device dax pages
when mapping"), this also leaks device-DAX folio references, mapcounts
and RSS accounting, resulting in "Bad rss-counter state" warnings on exit.

Allow PUD leaves in get_old_pud(). If a PUD leaf cannot be moved as a
whole or the move fails, split it. This unmaps it with proper accounting
instead of treating the leaf entry as a PMD table.

Keep using pud_trans_huge() for now, in the future this code should be
cleaned up to just check for PUD leaves.

Fix generated by an LLM and sent by Josef Bacik in private.

Fixes: a00cc7d9dd93 ("mm, x86: add support for PUD-sized transparent hugepages")
Cc: stable@vger.kernel.org
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
[ david: fallthrough after splitting ]
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Link: https://patch.msgid.link/20261031-fixes_core-v3-32-d6437171ecc3@kernel.org
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/mremap.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 91f0865f6aeea..1237718848f5c 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -87,7 +87,7 @@ static pud_t *get_old_pud(struct mm_struct *mm, unsigned long addr)
 		return NULL;
 
 	pud = pud_offset(p4d, addr);
-	if (pud_none_or_clear_bad(pud))
+	if (!pud_trans_huge(*pud) && pud_none_or_clear_bad(pud))
 		return NULL;
 
 	return pud;
@@ -847,11 +847,10 @@ unsigned long move_page_tables(struct pagetable_move_control *pmc)
 		if (!new_pud)
 			break;
 		if (pud_trans_huge(*old_pud)) {
-			if (extent == HPAGE_PUD_SIZE) {
-				move_pgt_entry(pmc, HPAGE_PUD, old_pud, new_pud);
-				/* We ignore and continue on error? */
+			if (extent == HPAGE_PUD_SIZE &&
+			    move_pgt_entry(pmc, HPAGE_PUD, old_pud, new_pud))
 				continue;
-			}
+			split_huge_pud(pmc->old, old_pud, pmc->old_addr);
 		} else if (IS_ENABLED(CONFIG_HAVE_MOVE_PUD) && extent == PUD_SIZE) {
 			if (move_pgt_entry(pmc, NORMAL_PUD, old_pud, new_pud))
 				continue;
-- 
2.53.0