linux-kernel - [PATCH] highmem: fix checks in __kmap_local_sched

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Date:   Sun,  3 Apr 2022 16:51:59 -0700
From:   Max Filippov <jcmvbkbc@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Max Filippov <jcmvbkbc@...il.com>
Subject: [PATCH] highmem: fix checks in __kmap_local_sched_{in,out}

When CONFIG_DEBUG_KMAP_LOCAL is enabled __kmap_local_sched_{in,out}
check that even slots in the tsk->kmap_ctrl.pteval are unmapped. The
slots are initialized with 0 value, but the check is done with pte_none.
0 pte however does not necessarily mean that pte_none will return true.
e.g. on xtensa it returns false, resulting in the following runtime
warnings:

 WARNING: CPU: 0 PID: 101 at mm/highmem.c:627 __kmap_local_sched_out+0x51/0x108
 CPU: 0 PID: 101 Comm: touch Not tainted 5.17.0-rc7-00010-gd3a1cdde80d2-dirty #13
 Call Trace:
   dump_stack+0xc/0x40
   __warn+0x8f/0x174
   warn_slowpath_fmt+0x48/0xac
   __kmap_local_sched_out+0x51/0x108
   __schedule+0x71a/0x9c4
   preempt_schedule_irq+0xa0/0xe0
   common_exception_return+0x5c/0x93
   do_wp_page+0x30e/0x330
   handle_mm_fault+0xa70/0xc3c
   do_page_fault+0x1d8/0x3c4
   common_exception+0x7f/0x7f

 WARNING: CPU: 0 PID: 101 at mm/highmem.c:664 __kmap_local_sched_in+0x50/0xe0
 CPU: 0 PID: 101 Comm: touch Tainted: G        W         5.17.0-rc7-00010-gd3a1cdde80d2-dirty #13
 Call Trace:
   dump_stack+0xc/0x40
   __warn+0x8f/0x174
   warn_slowpath_fmt+0x48/0xac
   __kmap_local_sched_in+0x50/0xe0
   finish_task_switch$isra$0+0x1ce/0x2f8
   __schedule+0x86e/0x9c4
   preempt_schedule_irq+0xa0/0xe0
   common_exception_return+0x5c/0x93
   do_wp_page+0x30e/0x330
   handle_mm_fault+0xa70/0xc3c
   do_page_fault+0x1d8/0x3c4
   common_exception+0x7f/0x7f

Fix it by replacing !pte_none(pteval) with pte_val(pteval) != 0.

Fixes: 5fbda3ecd14a ("sched: highmem: Store local kmaps in task struct")
Signed-off-by: Max Filippov <jcmvbkbc@...il.com>
---
 mm/highmem.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/highmem.c b/mm/highmem.c
index 762679050c9a..916b66e0776c 100644
--- a/mm/highmem.c
+++ b/mm/highmem.c
@@ -624,7 +624,7 @@ void __kmap_local_sched_out(void)
 
 		/* With debug all even slots are unmapped and act as guard */
 		if (IS_ENABLED(CONFIG_DEBUG_KMAP_LOCAL) && !(i & 0x01)) {
-			WARN_ON_ONCE(!pte_none(pteval));
+			WARN_ON_ONCE(pte_val(pteval) != 0);
 			continue;
 		}
 		if (WARN_ON_ONCE(pte_none(pteval)))
@@ -661,7 +661,7 @@ void __kmap_local_sched_in(void)
 
 		/* With debug all even slots are unmapped and act as guard */
 		if (IS_ENABLED(CONFIG_DEBUG_KMAP_LOCAL) && !(i & 0x01)) {
-			WARN_ON_ONCE(!pte_none(pteval));
+			WARN_ON_ONCE(pte_val(pteval) != 0);
 			continue;
 		}
 		if (WARN_ON_ONCE(pte_none(pteval)))
-- 
2.30.2