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

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Date:   Thu, 18 Mar 2021 16:08:39 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Oliver Sang <oliver.sang@...el.com>,
        Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>,
        David Sterba <dsterba@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "hch@...radead.org" <hch@...radead.org>,
        Christoph Hellwig <hch@....de>,
        LKML <linux-kernel@...r.kernel.org>,
        "lkp@...ts.01.org" <lkp@...ts.01.org>,
        "lkp@...el.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 Wed, Mar 17, 2021 at 02:44:41PM +0100, Thomas Gleixner wrote:
> 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.

Ok yea I was looking at this backwards.

> 
> > 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")
> 

Patch sent.

Thanks for setting me straight,
Ira