[PATCH 6.12.y] mm/mremap: fix moving PUD leaves

[Sasha Levin <sashal@kernel.org>]

From: Josef Bacik <josef@toxicpanda.com>

[ Upstream commit 440c8779757f09f290770b1d361dc1516a9d0b3e ]

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>
[ sashal: Adapted pagetable_move_control-based calls to individual arguments while retaining explicit pud_devmap() checks. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 mm/mremap.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 8b507e228a274..d357d36c1157b 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -47,7 +47,8 @@ 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_devmap(*pud) &&
+	    pud_none_or_clear_bad(pud))
 		return NULL;
 
 	return pud;
@@ -624,12 +625,11 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
 		if (!new_pud)
 			break;
 		if (pud_trans_huge(*old_pud) || pud_devmap(*old_pud)) {
-			if (extent == HPAGE_PUD_SIZE) {
-				move_pgt_entry(HPAGE_PUD, vma, old_addr, new_addr,
-					       old_pud, new_pud, need_rmap_locks);
-				/* We ignore and continue on error? */
+			if (extent == HPAGE_PUD_SIZE &&
+			    move_pgt_entry(HPAGE_PUD, vma, old_addr, new_addr,
+					   old_pud, new_pud, need_rmap_locks))
 				continue;
-			}
+			split_huge_pud(vma, old_pud, old_addr);
 		} else if (IS_ENABLED(CONFIG_HAVE_MOVE_PUD) &&
 			   extent == PUD_SIZE && uffd_supports_move) {
 
-- 
2.53.0