+ mm-make-split_pte_ptlocks-depend-on-smp.patch added to mm-hotfixes-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm: make SPLIT_PTE_PTLOCKS depend on SMP
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-make-split_pte_ptlocks-depend-on-smp.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-make-split_pte_ptlocks-depend-on-smp.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Guenter Roeck <linux@roeck-us.net>
Subject: mm: make SPLIT_PTE_PTLOCKS depend on SMP
Date: Tue, 24 Sep 2024 08:42:05 -0700

SPLIT_PTE_PTLOCKS depends on "NR_CPUS >= 4".  Unfortunately, that
evaluates to true if there is no NR_CPUS configuration option.  This
results in CONFIG_SPLIT_PTE_PTLOCKS=y for mac_defconfig.  This in turn
causes the m68k "q800" and "virt" machines to crash in qemu if debugging
options are enabled.

Making CONFIG_SPLIT_PTE_PTLOCKS dependent on the existence of NR_CPUS does
not work since a dependency on the existence of a numeric Kconfig entry
always evaluates to false.  Example:

config HAVE_NO_NR_CPUS
       def_bool y
       depends on !NR_CPUS

After adding this to a Kconfig file, "make defconfig" includes:
$ grep NR_CPUS .config
CONFIG_NR_CPUS=64
CONFIG_HAVE_NO_NR_CPUS=y

Defining NR_CPUS for m68k does not help either since many architectures
define NR_CPUS only for SMP configurations.

Make SPLIT_PTE_PTLOCKS depend on SMP instead to solve the problem.

Link: https://lkml.kernel.org/r/20240924154205.1491376-1-linux@roeck-us.net
Fixes: 394290cba966 ("mm: turn USE_SPLIT_PTE_PTLOCKS / USE_SPLIT_PTE_PTLOCKS into Kconfig options")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: David Hildenbrand <david@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/Kconfig |    1 +
 1 file changed, 1 insertion(+)

--- a/mm/Kconfig~mm-make-split_pte_ptlocks-depend-on-smp
+++ a/mm/Kconfig
@@ -595,6 +595,7 @@ config ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE
 config SPLIT_PTE_PTLOCKS
 	def_bool y
 	depends on MMU
+	depends on SMP
 	depends on NR_CPUS >= 4
 	depends on !ARM || CPU_CACHE_VIPT
 	depends on !PARISC || PA20
_

Patches currently in -mm which might be from linux@roeck-us.net are

mm-make-split_pte_ptlocks-depend-on-smp.patch