linux-kernel - Re: [mm/highmem] 61b205f579: WARNING:at_mm/highmem.c:#__kmap_local_sched

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Date:   Wed, 17 Mar 2021 14:44:41 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ira Weiny <ira.weiny@...el.com>,
        Oliver Sang <oliver.sang@...el.com>
Cc:     Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>,
        David Sterba <dsterba@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "hch\@infradead.org" <hch@...radead.org>,
        Christoph Hellwig <hch@....de>,
        LKML <linux-kernel@...r.kernel.org>,
        "lkp\@lists.01.org" <lkp@...ts.01.org>,
        "lkp\@intel.com" <lkp@...el.com>,
        Xing Zhengjun <zhengjun.xing@...ux.intel.com>
Subject: Re: [mm/highmem] 61b205f579: WARNING:at_mm/highmem.c:#__kmap_local_sched_out

On Tue, Mar 16 2021 at 00:37, Ira Weiny wrote:
>
> I think I see the issue.  I think this is an invalid configuration.
>
> 00:26:43 > grep DEBUG_KMAP config-5.11.0-rc7-00002-g61b205f57991 
> CONFIG_DEBUG_KMAP_LOCAL=y
> CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP=y
>
> 00:26:48 > grep DEBUG_HIGHMEM config-5.11.0-rc7-00002-g61b205f57991 
> # CONFIG_DEBUG_HIGHMEM is not set

No. It's valid so you can test that crap on 64bit.

> DEBUG_KMAP_LOCAL causes guard pages to be added to the kmap_ctrl array.  But
> DEBUG_HIGHMEM is used in __kmap_local_sched_out() to check the guard pages.
>
> DEBUG_HIGHMEM is supposed to select DEBUG_KMAP_LOCAL...  but apparently that
> did not happen when this configuration was made.
>
> I still have not hit this condition in my testing.  Could you ensure that
> DEBUG_HIGMEM is set and rerun the test to see if I am correct?

Why? The whole point of CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP is to allow
testing of this kmap_local/atomic stuff w/o HIGHMEM, e.g. on 64 bit.

> Thomas wouldn't the following enable checks make more sense?  Or perhaps be
> more consistent with the processing of kmap_ctrl?

Neither nor. It's simply required to make DEBUG_KMAP_LOCAL=y &&
HIGHMEM=n case work.

Please add:
Fixes: 0e91a0c6984c ("mm/highmem: Provide CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP")

which should have contained exactly that change:

> -               if (IS_ENABLED(CONFIG_DEBUG_HIGHMEM) && !(i & 0x01)) {
> +               if (IS_ENABLED(CONFIG_DEBUG_KMAP_LOCAL) && !(i & 0x01)) {
>                         WARN_ON_ONCE(!pte_none(pteval));
>                         continue;
>                 }
> @@ -654,7 +654,7 @@ void __kmap_local_sched_in(void)
>                 int idx;
>  
>                 /* With debug all even slots are unmapped and act as guard */
> -               if (IS_ENABLED(CONFIG_DEBUG_HIGHMEM) && !(i & 0x01)) {
> +               if (IS_ENABLED(CONFIG_DEBUG_KMAP_LOCAL) && !(i & 0x01)) {
>                         WARN_ON_ONCE(!pte_none(pteval));
>                         continue;
>                 }

Thanks,

        tglx