Re: [PATCH 1/2] mm/page_alloc: Access lists in 'struct per_cpu_pages' indirectly

[Marcelo Tosatti]

On Tue, Feb 08, 2022 at 11:07:49AM +0100, Nicolas Saenz Julienne wrote:
> In preparation to adding remote per-cpu page list drain support, let's
> bundle 'struct per_cpu_pages's' page lists and page count into a new
> structure: 'struct pcplists', and have all code access it indirectly
> through a pointer. It'll be used by upcoming patches in order to
> maintain multiple instances of 'pcplists' and switch the pointer
> atomically.
> 
> The 'struct pcplists' instance lives inside 'struct per_cpu_pages', and
> shouldn't be accessed directly. It is setup as such since these
> structures are used during early boot when no memory allocation is
> possible and to simplify memory hotplug code paths.
> 
> free_pcppages_bulk() and __rmqueue_pcplist()'s function signatures
> change a bit so as to accommodate these changes without affecting
> performance.
> 
> No functional change intended.
> 
> Signed-off-by: Nicolas Saenz Julienne <nsaenzju@xxxxxxxxxx>
> ---
> 
> Changes since RFC:
>  - Add more info in commit message.
>  - Removed __private attribute, in hindsight doesn't really fit what
>    we're doing here.
>  - Use raw_cpu_ptr() where relevant to avoid warnings.
> 
>  include/linux/mmzone.h | 10 +++--
>  mm/page_alloc.c        | 87 +++++++++++++++++++++++++-----------------
>  mm/vmstat.c            |  6 +--
>  3 files changed, 62 insertions(+), 41 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 3fff6deca2c0..b4cb85d9c6e8 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -381,7 +381,6 @@ enum zone_watermarks {
>  
>  /* Fields and list protected by pagesets local_lock in page_alloc.c */
>  struct per_cpu_pages {
> -	int count;		/* number of pages in the list */
>  	int high;		/* high watermark, emptying needed */
>  	int batch;		/* chunk size for buddy add/remove */
>  	short free_factor;	/* batch scaling factor during free */
> @@ -389,8 +388,13 @@ struct per_cpu_pages {
>  	short expire;		/* When 0, remote pagesets are drained */
>  #endif
>  
> -	/* Lists of pages, one per migrate type stored on the pcp-lists */
> -	struct list_head lists[NR_PCP_LISTS];
> +	struct pcplists *lp;
> +	struct pcplists {
> +		/* Number of pages in the pcplists */
> +		int count;
> +		/* Lists of pages, one per migrate type stored on the pcp-lists */
> +		struct list_head lists[NR_PCP_LISTS];
> +	} __pcplists; /* Do not access directly */
>  };

Perhaps its useful to add something like: "should be acessed through ..."

Looks good otherwise.