Re: [PATCH] mm: check nr_initialised with PAGES_PER_SECTION directly in defer_init()

[Alexander Duyck <alexander.h.duyck@linux.intel.com>]

On Thu, 2018-11-22 at 17:48 +0800, Wei Yang wrote:
> When DEFERRED_STRUCT_PAGE_INIT is configured, only the first section of
> each node's highest zone is initialized before defer stage.
> 
> static_init_pgcnt is used to store the number of pages like this:
> 
>     pgdat->static_init_pgcnt = min_t(unsigned long, PAGES_PER_SECTION,
>                                               pgdat->node_spanned_pages);
> 
> because we don't want to overflow zone's range.
> 
> But this is not necessary, since defer_init() is called like this:
> 
>   memmap_init_zone()
>     for pfn in [start_pfn, end_pfn)
>       defer_init(pfn, end_pfn)
> 
> In case (pgdat->node_spanned_pages < PAGES_PER_SECTION), the loop would
> stop before calling defer_init().
> 
> BTW, comparing PAGES_PER_SECTION with node_spanned_pages is not correct,
> since nr_initialised is zone based instead of node based. Even
> node_spanned_pages is bigger than PAGES_PER_SECTION, its highest zone
> would have pages less than PAGES_PER_SECTION.
> 
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>

This all seems to make sense to me, and appears to be a valid
improvement.

Reviewed-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>

> ---
>  include/linux/mmzone.h |  2 --
>  mm/page_alloc.c        | 13 ++++++-------
>  2 files changed, 6 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index ae7a830a21eb..68d7b558924b 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -692,8 +692,6 @@ typedef struct pglist_data {
>  	 * is the first PFN that needs to be initialised.
>  	 */
>  	unsigned long first_deferred_pfn;
> -	/* Number of non-deferred pages */
> -	unsigned long static_init_pgcnt;
>  #endif /* CONFIG_DEFERRED_STRUCT_PAGE_INIT */
>  
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 76e12179cd5e..b542d82400cf 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -326,8 +326,13 @@ defer_init(int nid, unsigned long pfn, unsigned long end_pfn)
>  	/* Always populate low zones for address-constrained allocations */
>  	if (end_pfn < pgdat_end_pfn(NODE_DATA(nid)))
>  		return false;
> +
> +	/*
> +	 * We start only with one section of pages, more pages are added as
> +	 * needed until the rest of deferred pages are initialized.
> +	 */
>  	nr_initialised++;
> -	if ((nr_initialised > NODE_DATA(nid)->static_init_pgcnt) &&
> +	if ((nr_initialised > PAGES_PER_SECTION) &&
>  	    (pfn & (PAGES_PER_SECTION - 1)) == 0) {
>  		NODE_DATA(nid)->first_deferred_pfn = pfn;
>  		return true;
> @@ -6451,12 +6456,6 @@ static void __ref alloc_node_mem_map(struct pglist_data *pgdat) { }
>  #ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
>  static inline void pgdat_set_deferred_range(pg_data_t *pgdat)
>  {
> -	/*
> -	 * We start only with one section of pages, more pages are added as
> -	 * needed until the rest of deferred pages are initialized.
> -	 */
> -	pgdat->static_init_pgcnt = min_t(unsigned long, PAGES_PER_SECTION,
> -						pgdat->node_spanned_pages);
>  	pgdat->first_deferred_pfn = ULONG_MAX;
>  }
>  #else