FAILED: patch "[PATCH] mm/mremap: recheck PUDs under the page table lock" failed to apply to 5.15-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 5.15-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-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 402882943dfa8bab5c2bcc66189f9fb2828f248e
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261109202338-failed-5.15-402882943dfa@kernel.org' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Thanks,
Sasha

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

From 402882943dfa8bab5c2bcc66189f9fb2828f248e Mon Sep 17 00:00:00 2001
From: "David Hildenbrand (Arm)" <david@kernel.org>
Date: Sat, 31 Oct 2026 10:40:00 +0100
Subject: [PATCH] mm/mremap: recheck PUDs under the page table lock

Once get_old_pud() is fixed to accept PUD leaves, unmap_mapping_range() can
clear a leaf between the lookup and the move. Holding mmap_lock in write
mode does not prevent this.

If the zap happens before move_page_tables() tests the entry, an empty
PUD can reach move_normal_pud(), where pud_populate() turns it into a
present page table entry. If it happens after the test, move_huge_pud()
can instead try to move an empty entry, triggering CONFIG_DEBUG_VM
warnings on powerpc.

Recheck the source under the page table lock in both helpers. Treat an
empty source in move_normal_pud() as a completed move, and use
__pud_trans_huge_lock() in move_huge_pud().

In the future we should likely have the code lookup a PUD/PMD lockless
once, instead of multiple times, because one of the problematic bits
here is the:

	pud_trans_huge(*old_pud)

Fixes: 0881ace292b6 ("mm/mremap: use pmd/pud_poplulate to update page table entries")
Cc: stable@vger.kernel.org
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Link: https://patch.msgid.link/20261031-fixes_core-v3-31-d6437171ecc3@kernel.org
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/mremap.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 16e736c72d9cb..91f0865f6aeea 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -461,14 +461,18 @@ static bool move_normal_pud(struct pagetable_move_control *pmc,
 	if (new_ptl != old_ptl)
 		spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
 
-	/* Clear the pud */
 	pud = *old_pud;
+	if (unlikely(pud_none(pud)))
+		goto out_unlock;
+
+	/* Clear the pud */
 	pud_clear(old_pud);
 
 	VM_BUG_ON(!pud_none(*new_pud));
 
 	pud_populate(mm, new_pud, pud_pgtable(pud));
 	flush_tlb_range(vma, pmc->old_addr, pmc->old_addr + PUD_SIZE);
+out_unlock:
 	if (new_ptl != old_ptl)
 		spin_unlock(new_ptl);
 	spin_unlock(old_ptl);
@@ -503,7 +507,9 @@ static bool move_huge_pud(struct pagetable_move_control *pmc,
 	 * We don't have to worry about the ordering of src and dst
 	 * ptlocks because exclusive mmap_lock prevents deadlock.
 	 */
-	old_ptl = pud_lock(mm, old_pud);
+	old_ptl = __pud_trans_huge_lock(old_pud, vma);
+	if (unlikely(!old_ptl))
+		return false;
 	new_ptl = pud_lockptr(mm, new_pud);
 	if (new_ptl != old_ptl)
 		spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
-- 
2.53.0