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

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Date:   Mon, 04 Apr 2022 17:12:01 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Max Filippov <jcmvbkbc@...il.com>
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: Re: [PATCH] highmem: fix checks in __kmap_local_sched_{in,out}

On Sun, Apr 03 2022 at 16:51, Max Filippov wrote:

> 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.

Oops. Sorry.

> Fixes: 5fbda3ecd14a ("sched: highmem: Store local kmaps in task struct")
> Signed-off-by: Max Filippov <jcmvbkbc@...il.com>

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>