Re: [PATCH] mm/memcontrol: avoid unnecessary PageTransHuge() when counting compound page

[Michal Hocko <mhocko@suse.com>]

On Sun 05-05-19 14:40:57, Yafang Shao wrote:
> If CONFIG_TRANSPARENT_HUGEPAGE is not set, hpage_nr_pages() is always 1;
> if CONFIG_TRANSPARENT_HUGEPAGE is set, hpage_nr_pages() will
> call PageTransHuge() to judge whether the page is compound page or not.
> So we can use the result of hpage_nr_pages() to avoid uneccessary
> PageTransHuge().

The changelog doesn't describe motivation. Does this result in a better
code/performance?
 
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> ---
>  mm/memcontrol.c | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 2535e54..65c6f7c 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6306,7 +6306,6 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage)
>  {
>  	struct mem_cgroup *memcg;
>  	unsigned int nr_pages;
> -	bool compound;
>  	unsigned long flags;
>  
>  	VM_BUG_ON_PAGE(!PageLocked(oldpage), oldpage);
> @@ -6328,8 +6327,7 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage)
>  		return;
>  
>  	/* Force-charge the new page. The old one will be freed soon */
> -	compound = PageTransHuge(newpage);
> -	nr_pages = compound ? hpage_nr_pages(newpage) : 1;
> +	nr_pages = hpage_nr_pages(newpage);
>  
>  	page_counter_charge(&memcg->memory, nr_pages);
>  	if (do_memsw_account())
> @@ -6339,7 +6337,7 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage)
>  	commit_charge(newpage, memcg, false);
>  
>  	local_irq_save(flags);
> -	mem_cgroup_charge_statistics(memcg, newpage, compound, nr_pages);
> +	mem_cgroup_charge_statistics(memcg, newpage, nr_pages > 1, nr_pages);
>  	memcg_check_events(memcg, newpage);
>  	local_irq_restore(flags);
>  }
> @@ -6533,6 +6531,7 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
>  	struct mem_cgroup *memcg, *swap_memcg;
>  	unsigned int nr_entries;
>  	unsigned short oldid;
> +	bool compound;
>  
>  	VM_BUG_ON_PAGE(PageLRU(page), page);
>  	VM_BUG_ON_PAGE(page_count(page), page);
> @@ -6553,8 +6552,9 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
>  	 */
>  	swap_memcg = mem_cgroup_id_get_online(memcg);
>  	nr_entries = hpage_nr_pages(page);
> +	compound = nr_entries > 1;
>  	/* Get references for the tail pages, too */
> -	if (nr_entries > 1)
> +	if (compound)
>  		mem_cgroup_id_get_many(swap_memcg, nr_entries - 1);
>  	oldid = swap_cgroup_record(entry, mem_cgroup_id(swap_memcg),
>  				   nr_entries);
> @@ -6579,8 +6579,7 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
>  	 * only synchronisation we have for updating the per-CPU variables.
>  	 */
>  	VM_BUG_ON(!irqs_disabled());
> -	mem_cgroup_charge_statistics(memcg, page, PageTransHuge(page),
> -				     -nr_entries);
> +	mem_cgroup_charge_statistics(memcg, page, compound, -nr_entries);
>  	memcg_check_events(memcg, page);
>  
>  	if (!mem_cgroup_is_root(memcg))
> -- 
> 1.8.3.1

-- 
Michal Hocko
SUSE Labs