linux-kernel - Re: [PATCH v2 1/1] mm: Correct misleading comment on mmap_lock field in mm

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Message-ID: <9305bdaf-9455-4c26-befb-471466f952ab@lucifer.local>
Date: Wed, 6 Aug 2025 20:07:24 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "Adrian Huang (Lenovo)" <adrianhuang0701@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>, Liam.Howlett@...cle.com,
        Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Feng Tang <feng.79.tang@...il.com>, ahuang12@...ovo.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] mm: Correct misleading comment on mmap_lock field
 in mm_struct

On Wed, Aug 06, 2025 at 10:59:06PM +0800, Adrian Huang (Lenovo) wrote:
> The comment previously described the offset of mmap_lock as 0x120 (hex),
> which is misleading. The correct offset is 56 bytes (decimal) from the
> last cache line boundary. 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.
>
> Signed-off-by: Adrian Huang (Lenovo) <adrianhuang0701@...il.com>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

> ---
> Changes in v2: Per Lorenzo's suggestion, use "56 bytes" instead of 120.
>
>  include/linux/mm_types.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 1ec273b06691..c9c3d0307f8c 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -1026,10 +1026,10 @@ struct mm_struct {
>  					     * counters
>  					     */
>  		/*
> -		 * With some kernel config, the current mmap_lock's offset
> -		 * inside 'mm_struct' is at 0x120, which is very optimal, as
> +		 * Typically the current mmap_lock's offset is 56 bytes from
> +		 * the last cacheline boundary, 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
>