[PATCH] docs/mm: describe current criteria for enabling split page table lock for PTE tables

[PATCH] docs/mm: describe current criteria for enabling split page table lock for PTE tables

[Ethan Nelson-Moore]

The mm documentation regarding split page table lock for PTE tables
refers to the CONFIG_SPLIT_PTLOCK_CPUS config option, which was
superseded by CONFIG_SPLIT_PTE_PTLOCKS in commit 394290cba966 ("mm:
turn USE_SPLIT_PTE_PTLOCKS / USE_SPLIT_PTE_PTLOCKS into Kconfig
options"). Update the documentation to refer to the current option and
document the situations in which this feature is not supported.

Discovered while searching for CONFIG_* symbols referenced in the
kernel but not defined in any Kconfig file.

Signed-off-by: Ethan Nelson-Moore <enelsonmoore@gmail.com>
---
 Documentation/mm/split_page_table_lock.rst | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/Documentation/mm/split_page_table_lock.rst b/Documentation/mm/split_page_table_lock.rst
index cc3cd46abd1b..c9d16024543b 100644
--- a/Documentation/mm/split_page_table_lock.rst
+++ b/Documentation/mm/split_page_table_lock.rst
@@ -37,9 +37,12 @@ There are helpers to lock/unlock a table and other accessor functions:
  - pmd_lockptr()
 	returns pointer to PMD table lock;
 
-Split page table lock for PTE tables is enabled compile-time if
-CONFIG_SPLIT_PTLOCK_CPUS (usually 4) is less or equal to NR_CPUS.
-If split lock is disabled, all tables are guarded by mm->page_table_lock.
+Split page table lock for PTE tables is enabled compile-time (via
+CONFIG_SPLIT_PTE_PTLOCKS) if NR_CPUS is greater than or equal to 4 and an
+MMU is being used. However, it is not supported on ARM processors with
+virtually indexed, physically tagged caches, PA-RISC processors older than
+the PA-8000, or 32-bit SPARC processors. If split lock is disabled, all
+tables are guarded by mm->page_table_lock.
 
 Split page table lock for PMD tables is enabled, if it's enabled for PTE
 tables and the architecture supports it (see below).
-- 
2.43.0

Re: [PATCH] docs/mm: describe current criteria for enabling split page table lock for PTE tables

[Mike Rapoport]

On Sat, Jun 13, 2026 at 03:14:47PM -0700, Ethan Nelson-Moore wrote:
> The mm documentation regarding split page table lock for PTE tables
> refers to the CONFIG_SPLIT_PTLOCK_CPUS config option, which was
> superseded by CONFIG_SPLIT_PTE_PTLOCKS in commit 394290cba966 ("mm:
> turn USE_SPLIT_PTE_PTLOCKS / USE_SPLIT_PTE_PTLOCKS into Kconfig
> options"). Update the documentation to refer to the current option and
> document the situations in which this feature is not supported.
> 
> Discovered while searching for CONFIG_* symbols referenced in the
> kernel but not defined in any Kconfig file.
> 
> Signed-off-by: Ethan Nelson-Moore <enelsonmoore@gmail.com>
> ---
>  Documentation/mm/split_page_table_lock.rst | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/mm/split_page_table_lock.rst b/Documentation/mm/split_page_table_lock.rst
> index cc3cd46abd1b..c9d16024543b 100644
> --- a/Documentation/mm/split_page_table_lock.rst
> +++ b/Documentation/mm/split_page_table_lock.rst
> @@ -37,9 +37,12 @@ There are helpers to lock/unlock a table and other accessor functions:
>   - pmd_lockptr()
>  	returns pointer to PMD table lock;
>  
> -Split page table lock for PTE tables is enabled compile-time if
> -CONFIG_SPLIT_PTLOCK_CPUS (usually 4) is less or equal to NR_CPUS.
> -If split lock is disabled, all tables are guarded by mm->page_table_lock.
> +Split page table lock for PTE tables is enabled compile-time (via
> +CONFIG_SPLIT_PTE_PTLOCKS) if NR_CPUS is greater than or equal to 4 and an
> +MMU is being used. However, it is not supported on ARM processors with
> +virtually indexed, physically tagged caches, PA-RISC processors older than
> +the PA-8000, or 32-bit SPARC processors. If split lock is disabled, all

"it is not supported on some architectures" would be enough IMO.

> +tables are guarded by mm->page_table_lock.
>  
>  Split page table lock for PMD tables is enabled, if it's enabled for PTE
>  tables and the architecture supports it (see below).
> -- 
> 2.43.0
> 

-- 
Sincerely yours,
Mike.