[PATCH] mm/migrate: fix softlockup in do_pages_move() with huge page arrays

[<shao.mingyin@zte.com.cn>]

From: Shao Mingyin <shao.mingyin@zte.com.cn>

mm/migrate: fix softlockup in do_pages_move() with huge page arrays

do_pages_move() walks the user page array one page at a time.  For each
entry it performs two get_user() calls, an mmap_read_lock()/unlock()
around a folio walk, and an optional store_status(); none of them has to
sleep when the arrays stay resident and the rwsem is uncontended.
move_pages_and_store_status() returns early while the page list is
empty, so an array whose entries all resolve to the same node can run
the whole loop without any scheduling point.  nr_pages comes straight
from move_pages(), so a huge array can hold a CPU past the watchdog
threshold on a non-preemptible kernel; with KASAN and function tracing
enabled, ten million entries can exceed it.

Add cond_resched() so that long calls yield periodically.
do_pages_stat() has the same problem and is being fixed separately.  In
current mainline only alpha, hexagon and classic m68k can still select
PREEMPT_NONE, and they cannot enable CONFIG_NUMA, so this mainly
targets pre-v7.0 stable kernels.

Fixes: a49bd4d71637 ("mm, numa: rework do_pages_move")
Cc: stable@vger.kernel.org
Signed-off-by: Shao Mingyin <shao.mingyin@zte.com.cn>
---
 mm/migrate.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/migrate.c b/mm/migrate.c
index 875079464930..3122ecb85270 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2372,6 +2372,8 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
 		const void __user *p;
 		int node;
 
+		cond_resched();
+
 		err = -EFAULT;
 		if (in_compat_syscall()) {
 			compat_uptr_t cp;