FROMLIST: mm/mempolicy: fix unbalanced unlock in backported VMA check

No upstream commit exists for this commit.

The issue was introduced with backporting upstream commit 091c1dd
("mm/mempolicy: fix migrate_to_node() assuming there is at least one VMA
in a MM").

The backport incorrectly added unlock logic to a path where
mmap_read_lock() wasn't acquired, creating lock imbalance when no VMAs
are found.

This fixes the report:

WARNING: bad unlock balance detected!
6.6.79 #1 Not tainted
-------------------------------------
repro/9655 is trying to release lock (&mm->mmap_lock) at:
[<ffffffff81daa36f>] mmap_read_unlock include/linux/mmap_lock.h:173 [inline]
[<ffffffff81daa36f>] do_migrate_pages+0x59f/0x700 mm/mempolicy.c:1196
but there are no more locks to release!

other info that might help us debug this:
no locks held by repro/9655.

stack backtrace:
CPU: 1 PID: 9655 Comm: a Not tainted 6.6.79 #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0xd5/0x1b0 lib/dump_stack.c:106
 __lock_release kernel/locking/lockdep.c:5431 [inline]
 lock_release+0x4b1/0x680 kernel/locking/lockdep.c:5774
 up_read+0x12/0x20 kernel/locking/rwsem.c:1615
 mmap_read_unlock include/linux/mmap_lock.h:173 [inline]
 do_migrate_pages+0x59f/0x700 mm/mempolicy.c:1196
 kernel_migrate_pages+0x59b/0x780 mm/mempolicy.c:1665
 __do_sys_migrate_pages mm/mempolicy.c:1684 [inline]
 __se_sys_migrate_pages mm/mempolicy.c:1680 [inline]
 __x64_sys_migrate_pages+0x92/0xf0 mm/mempolicy.c:1680
 do_syscall_x64 arch/x86/entry/common.c:51 [inline]
 do_syscall_64+0x34/0xb0 arch/x86/entry/common.c:81
 entry_SYSCALL_64_after_hwframe+0x68/0xd2

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

Fixes: a13b2b9 ("mm/mempolicy: fix migrate_to_node() assuming there is at least one VMA in a MM")
Signed-off-by: Alexey Panov <apanov@astralinux.ru>
Signed-off-by: Wentao Guan <guanwentao@uniontech.com>

Author: Alexey Panov

mm/mempolicy.c

@@ -1072,7 +1072,6 @@ static long migrate_to_node(struct mm_struct *mm, int source, int dest,
 	VM_BUG_ON(!(flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)));
 	vma = find_vma(mm, 0);
 	if (unlikely(!vma)) {
-		mmap_read_unlock(mm);
 		return 0;
 	}