[PATCH 1/1] mm: Correct misleading comment on mmap_lock field in mm_struct

[PATCH 1/1] mm: Correct misleading comment on mmap_lock field in mm_struct

[Adrian Huang]

From: Adrian Huang <ahuang12@lenovo.com>

The comment previously described the offset of mmap_lock as 0x120 (hex),
which is misleading. The correct value is 120 (decimal), and using '0x120'
could confuse readers trying to understand why the count and owner fields
reside in separate cachelines.

This change also removes an unnecessary space for improved formatting.

Fixes: 2e3025434a6b ("mm: relocate 'write_protect_seq' in struct mm_struct")
Signed-off-by: Adrian Huang <ahuang12@lenovo.com>
---
 include/linux/mm_types.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 1ec273b06691..ec90bbf22e2b 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1027,9 +1027,9 @@ struct mm_struct {
 					     */
 		/*
 		 * With some kernel config, the current mmap_lock's offset
-		 * inside 'mm_struct' is at 0x120, which is very optimal, as
+		 * inside 'mm_struct' is at 120, which is very optimal, as
 		 * its two hot fields 'count' and 'owner' sit in 2 different
-		 * cachelines,  and when mmap_lock is highly contended, both
+		 * cachelines, and when mmap_lock is highly contended, both
 		 * of the 2 fields will be accessed frequently, current layout
 		 * will help to reduce cache bouncing.
 		 *
-- 
2.34.1

Re: [PATCH 1/1] mm: Correct misleading comment on mmap_lock field in mm_struct

[Vlastimil Babka]

On 8/5/25 10:47 AM, Adrian Huang wrote:
> From: Adrian Huang <ahuang12@lenovo.com>
> 
> The comment previously described the offset of mmap_lock as 0x120 (hex),
> which is misleading. The correct value is 120 (decimal), and using '0x120'
> could confuse readers trying to understand why the count and owner fields
> reside in separate cachelines.
> 
> This change also removes an unnecessary space for improved formatting.
> 
> Fixes: 2e3025434a6b ("mm: relocate 'write_protect_seq' in struct mm_struct")
> Signed-off-by: Adrian Huang <ahuang12@lenovo.com>

That seems all true so

Acked-by: Vlastimil Babka <vbabka@suse.cz>

But I wonder why we just hope this remains true "with some kernel
config" and don't employ some explicit alignment to make sure it's true
(except perhaps with some debug options like lockdep bloating the
structures, but we don't care about perfmance in such configs).

> ---
>  include/linux/mm_types.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 1ec273b06691..ec90bbf22e2b 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -1027,9 +1027,9 @@ struct mm_struct {
>  					     */
>  		/*
>  		 * With some kernel config, the current mmap_lock's offset
> -		 * inside 'mm_struct' is at 0x120, which is very optimal, as
> +		 * inside 'mm_struct' is at 120, which is very optimal, as
>  		 * its two hot fields 'count' and 'owner' sit in 2 different
> -		 * cachelines,  and when mmap_lock is highly contended, both
> +		 * cachelines, and when mmap_lock is highly contended, both
>  		 * of the 2 fields will be accessed frequently, current layout
>  		 * will help to reduce cache bouncing.
>  		 *

Re: [PATCH 1/1] mm: Correct misleading comment on mmap_lock field in mm_struct

[Lorenzo Stoakes]

On Tue, Aug 05, 2025 at 04:47:26PM +0800, Adrian Huang wrote:
> From: Adrian Huang <ahuang12@lenovo.com>
>
> The comment previously described the offset of mmap_lock as 0x120 (hex),
> which is misleading. The correct value is 120 (decimal), and using '0x120'
> could confuse readers trying to understand why the count and owner fields
> reside in separate cachelines.

This isn't the case on my system, pahole puts it at 184, if you subtract the
first field + cache line padding at the start of the structure, then 120 from
there... I guess?

I think it'd be better to say 'offset 56 bytes past the start of the cache
line' rather than to give specific values.

E.g.:

/*
 * Typically the current mmap_lock's offset is 56 bytes from the last cache
   line boundary, which is very optimal... etc. etc.
 ...
 */

>
> This change also removes an unnecessary space for improved formatting.
>
> Fixes: 2e3025434a6b ("mm: relocate 'write_protect_seq' in struct mm_struct")

There's no need for fixes for a comment change.

> Signed-off-by: Adrian Huang <ahuang12@lenovo.com>

Thanks! You're sending from a gmail account though, please make the email you
send from and signed off the same, or otherwise we don't know whether this is
the real you :)

> ---
>  include/linux/mm_types.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 1ec273b06691..ec90bbf22e2b 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -1027,9 +1027,9 @@ struct mm_struct {
>  					     */
>  		/*
>  		 * With some kernel config, the current mmap_lock's offset
> -		 * inside 'mm_struct' is at 0x120, which is very optimal, as
> +		 * inside 'mm_struct' is at 120, which is very optimal, as
>  		 * its two hot fields 'count' and 'owner' sit in 2 different
> -		 * cachelines,  and when mmap_lock is highly contended, both
> +		 * cachelines, and when mmap_lock is highly contended, both
>  		 * of the 2 fields will be accessed frequently, current layout
>  		 * will help to reduce cache bouncing.
>  		 *
> --
> 2.34.1
>

Cheers, Lorenzo