linux-kernel - [PATCH v3] mm: Make SPLIT_PTE

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Message-ID: <20240924154205.1491376-1-linux@roeck-us.net>
Date: Tue, 24 Sep 2024 08:42:05 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Guenter Roeck <linux@...ck-us.net>,
	David Hildenbrand <david@...hat.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: [PATCH v3] mm: Make SPLIT_PTE_PTLOCKS depend on SMP

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.

Fixes: 394290cba966 ("mm: turn USE_SPLIT_PTE_PTLOCKS / USE_SPLIT_PTE_PTLOCKS into Kconfig options")
Cc: David Hildenbrand <david@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
Signed-off-by: Guenter Roeck <linux@...ck-us.net>
---
v3: Make SPLIT_PTE_PTLOCKS depend on SMP instead of defining NR_CPUS
    for m68k.
    Drop David's Acked-by: tag.

v2: Instead of trying to make SPLIT_PTE_PTLOCKS depend on the 
    existence of NR_CPUS, define NR_CPUS for m68k.
    https://lore.kernel.org/lkml/20240923235617.1584056-1-linux@roeck-us.net/

v1: https://lore.kernel.org/lkml/20240923142533.1197982-1-linux@roeck-us.net/

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

diff --git a/mm/Kconfig b/mm/Kconfig
index e85164c2812b..33fa51d608dc 100644
--- a/mm/Kconfig
+++ b/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
-- 
2.45.2