linux-kernel - Re: [PATCH] mm: Make SPLIT_PTE_PTLOCKS depend on the existence of NR

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Message-ID: <203f0d01-d25e-4436-b769-b89edb1b57d9@roeck-us.net>
Date: Mon, 23 Sep 2024 16:52:49 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: David Hildenbrand <david@...hat.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH] mm: Make SPLIT_PTE_PTLOCKS depend on the existence of
 NR_CPUS

On 9/23/24 15:08, Guenter Roeck wrote:
> On 9/23/24 08:23, David Hildenbrand wrote:
>> On 23.09.24 16:25, Guenter Roeck wrote:
>>> SPLIT_PTE_PTLOCKS already depends on "NR_CPUS >= 4", but that evaluates
>>> to true if there is no NR_CPUS configuration option (such as for m68k).
>>> This results in CONFIG_SPLIT_PTE_PTLOCKS=y for mac_defconfig.
>>> This in turn causes the m68k "q800" machine to crash in qemu.
>>
>> Oh, that's why my compile tests still worked ... I even removed the additional NR_CPUS check, assuming it's not required ...
>>
>> Thanks for debugging and fixing!
>>
>> Acked-by: David Hildenbrand <david@...hat.com>
>>
> 
> Apparently it wasn't that simple :-(. 0-day reports a build failure
> with s390 builds.
> 
> arch/s390/mm/gmap.c:357:16: error: implicit declaration of function 'pmd_pgtable_page'.
> 
> Turns out that
>      depends on NR_CPUS && NR_CPUS >= 4
> 
> doesn't work and disables SPLIT_PTE_PTLOCKS even if NR_CPUS _is_ defined.
> I have no idea how to declare the dependency correctly.
> Sorry, I did not expect that.
> 

The only solution I found was to define NR_CPUS for m68k. That seems to be
the only architecture not defining it, so hopefully that is an acceptable
solution. I'll send v2 of the patch shortly.

Guenter