Re: [patch 4/4] mm: memcontrol: remove unnecessary PCG_USED pc->mem_cgroup valid flag

[Kamezawa Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>]

(2014/10/21 0:22), Johannes Weiner wrote:
> pc->mem_cgroup had to be left intact after uncharge for the final LRU
> removal, and !PCG_USED indicated whether the page was uncharged.  But
> since 0a31bc97c80c ("mm: memcontrol: rewrite uncharge API") pages are
> uncharged after the final LRU removal.  Uncharge can simply clear the
> pointer and the PCG_USED/PageCgroupUsed sites can test that instead.
> 
> Because this is the last page_cgroup flag, this patch reduces the
> memcg per-page overhead to a single pointer.
> 
> Signed-off-by: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>

awesome.
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>

> ---
>   include/linux/page_cgroup.h |  10 -----
>   mm/memcontrol.c             | 107 +++++++++++++++++---------------------------
>   2 files changed, 42 insertions(+), 75 deletions(-)
> 
> diff --git a/include/linux/page_cgroup.h b/include/linux/page_cgroup.h
> index 97536e685843..1289be6b436c 100644
> --- a/include/linux/page_cgroup.h
> +++ b/include/linux/page_cgroup.h
> @@ -1,11 +1,6 @@
>   #ifndef __LINUX_PAGE_CGROUP_H
>   #define __LINUX_PAGE_CGROUP_H
>   
> -enum {
> -	/* flags for mem_cgroup */
> -	PCG_USED = 0x01,	/* This page is charged to a memcg */
> -};
> -
>   struct pglist_data;
>   
>   #ifdef CONFIG_MEMCG
> @@ -19,7 +14,6 @@ struct mem_cgroup;
>    * then the page cgroup for pfn always exists.
>    */
>   struct page_cgroup {
> -	unsigned long flags;
>   	struct mem_cgroup *mem_cgroup;
>   };
>   
> @@ -39,10 +33,6 @@ static inline void page_cgroup_init(void)
>   
>   struct page_cgroup *lookup_page_cgroup(struct page *page);
>   
> -static inline int PageCgroupUsed(struct page_cgroup *pc)
> -{
> -	return !!(pc->flags & PCG_USED);
> -}
>   #else /* !CONFIG_MEMCG */
>   struct page_cgroup;
>   
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 1d66ac49e702..48d49c6b08d1 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1284,14 +1284,12 @@ struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct zone *zone)
>   
>   	pc = lookup_page_cgroup(page);
>   	memcg = pc->mem_cgroup;
> -
>   	/*
>   	 * Swapcache readahead pages are added to the LRU - and
> -	 * possibly migrated - before they are charged.  Ensure
> -	 * pc->mem_cgroup is sane.
> +	 * possibly migrated - before they are charged.
>   	 */
> -	if (!PageLRU(page) && !PageCgroupUsed(pc) && memcg != root_mem_cgroup)
> -		pc->mem_cgroup = memcg = root_mem_cgroup;
> +	if (!memcg)
> +		memcg = root_mem_cgroup;
>   
>   	mz = mem_cgroup_page_zoneinfo(memcg, page);
>   	lruvec = &mz->lruvec;
> @@ -2141,7 +2139,7 @@ void __mem_cgroup_begin_update_page_stat(struct page *page,
>   	pc = lookup_page_cgroup(page);
>   again:
>   	memcg = pc->mem_cgroup;
> -	if (unlikely(!memcg || !PageCgroupUsed(pc)))
> +	if (unlikely(!memcg))
>   		return;
>   	/*
>   	 * If this memory cgroup is not under account moving, we don't
> @@ -2154,7 +2152,7 @@ again:
>   		return;
>   
>   	move_lock_mem_cgroup(memcg, flags);
> -	if (memcg != pc->mem_cgroup || !PageCgroupUsed(pc)) {
> +	if (memcg != pc->mem_cgroup) {
>   		move_unlock_mem_cgroup(memcg, flags);
>   		goto again;
>   	}
> @@ -2186,7 +2184,7 @@ void mem_cgroup_update_page_stat(struct page *page,
>   
>   	pc = lookup_page_cgroup(page);
>   	memcg = pc->mem_cgroup;
> -	if (unlikely(!memcg || !PageCgroupUsed(pc)))
> +	if (unlikely(!memcg))
>   		return;
>   
>   	this_cpu_add(memcg->stat->count[idx], val);
> @@ -2525,9 +2523,10 @@ struct mem_cgroup *try_get_mem_cgroup_from_page(struct page *page)
>   	VM_BUG_ON_PAGE(!PageLocked(page), page);
>   
>   	pc = lookup_page_cgroup(page);
> -	if (PageCgroupUsed(pc)) {
> -		memcg = pc->mem_cgroup;
> -		if (memcg && !css_tryget_online(&memcg->css))
> +	memcg = pc->mem_cgroup;
> +
> +	if (memcg) {
> +		if (!css_tryget_online(&memcg->css))
>   			memcg = NULL;
>   	} else if (PageSwapCache(page)) {
>   		ent.val = page_private(page);
> @@ -2578,7 +2577,7 @@ static void commit_charge(struct page *page, struct mem_cgroup *memcg,
>   	struct page_cgroup *pc = lookup_page_cgroup(page);
>   	int isolated;
>   
> -	VM_BUG_ON_PAGE(PageCgroupUsed(pc), page);
> +	VM_BUG_ON_PAGE(pc->mem_cgroup, page);
>   	/*
>   	 * we don't need page_cgroup_lock about tail pages, becase they are not
>   	 * accessed by any other context at this point.
> @@ -2593,7 +2592,7 @@ static void commit_charge(struct page *page, struct mem_cgroup *memcg,
>   
>   	/*
>   	 * Nobody should be changing or seriously looking at
> -	 * pc->mem_cgroup and pc->flags at this point:
> +	 * pc->mem_cgroup at this point:
>   	 *
>   	 * - the page is uncharged
>   	 *
> @@ -2606,7 +2605,6 @@ static void commit_charge(struct page *page, struct mem_cgroup *memcg,
>   	 *   have the page locked
>   	 */
>   	pc->mem_cgroup = memcg;
> -	pc->flags = PCG_USED;
>   
>   	if (lrucare)
>   		unlock_page_lru(page, isolated);
> @@ -3120,37 +3118,22 @@ void __memcg_kmem_commit_charge(struct page *page, struct mem_cgroup *memcg,
>   		memcg_uncharge_kmem(memcg, 1 << order);
>   		return;
>   	}
> -	/*
> -	 * The page is freshly allocated and not visible to any
> -	 * outside callers yet.  Set up pc non-atomically.
> -	 */
>   	pc = lookup_page_cgroup(page);
>   	pc->mem_cgroup = memcg;
> -	pc->flags = PCG_USED;
>   }
>   
>   void __memcg_kmem_uncharge_pages(struct page *page, int order)
>   {
> -	struct mem_cgroup *memcg = NULL;
> -	struct page_cgroup *pc;
> -
> -
> -	pc = lookup_page_cgroup(page);
> -	if (!PageCgroupUsed(pc))
> -		return;
> -
> -	memcg = pc->mem_cgroup;
> -	pc->flags = 0;
> +	struct page_cgroup *pc = lookup_page_cgroup(page);
> +	struct mem_cgroup *memcg = pc->mem_cgroup;
>   
> -	/*
> -	 * We trust that only if there is a memcg associated with the page, it
> -	 * is a valid allocation
> -	 */
>   	if (!memcg)
>   		return;
>   
>   	VM_BUG_ON_PAGE(mem_cgroup_is_root(memcg), page);
> +
>   	memcg_uncharge_kmem(memcg, 1 << order);
> +	pc->mem_cgroup = NULL;
>   }
>   #else
>   static inline void memcg_unregister_all_caches(struct mem_cgroup *memcg)
> @@ -3168,21 +3151,16 @@ static inline void memcg_unregister_all_caches(struct mem_cgroup *memcg)
>    */
>   void mem_cgroup_split_huge_fixup(struct page *head)
>   {
> -	struct page_cgroup *head_pc = lookup_page_cgroup(head);
> -	struct page_cgroup *pc;
> -	struct mem_cgroup *memcg;
> +	struct page_cgroup *pc = lookup_page_cgroup(head);
>   	int i;
>   
>   	if (mem_cgroup_disabled())
>   		return;
>   
> -	memcg = head_pc->mem_cgroup;
> -	for (i = 1; i < HPAGE_PMD_NR; i++) {
> -		pc = head_pc + i;
> -		pc->mem_cgroup = memcg;
> -		pc->flags = head_pc->flags;
> -	}
> -	__this_cpu_sub(memcg->stat->count[MEM_CGROUP_STAT_RSS_HUGE],
> +	for (i = 1; i < HPAGE_PMD_NR; i++)
> +		pc[i].mem_cgroup = pc[0].mem_cgroup;
> +
> +	__this_cpu_sub(pc[0].mem_cgroup->stat->count[MEM_CGROUP_STAT_RSS_HUGE],
>   		       HPAGE_PMD_NR);
>   }
>   #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> @@ -3232,7 +3210,7 @@ static int mem_cgroup_move_account(struct page *page,
>   		goto out;
>   
>   	ret = -EINVAL;
> -	if (!PageCgroupUsed(pc) || pc->mem_cgroup != from)
> +	if (pc->mem_cgroup != from)
>   		goto out_unlock;
>   
>   	move_lock_mem_cgroup(from, &flags);
> @@ -3342,7 +3320,7 @@ static struct page_cgroup *lookup_page_cgroup_used(struct page *page)
>   	 * the first time, i.e. during boot or memory hotplug;
>   	 * or when mem_cgroup_disabled().
>   	 */
> -	if (likely(pc) && PageCgroupUsed(pc))
> +	if (likely(pc) && pc->mem_cgroup)
>   		return pc;
>   	return NULL;
>   }
> @@ -3360,10 +3338,8 @@ void mem_cgroup_print_bad_page(struct page *page)
>   	struct page_cgroup *pc;
>   
>   	pc = lookup_page_cgroup_used(page);
> -	if (pc) {
> -		pr_alert("pc:%p pc->flags:%lx pc->mem_cgroup:%p\n",
> -			 pc, pc->flags, pc->mem_cgroup);
> -	}
> +	if (pc)
> +		pr_alert("pc:%p pc->mem_cgroup:%p\n", pc, pc->mem_cgroup);
>   }
>   #endif
>   
> @@ -5330,7 +5306,7 @@ static enum mc_target_type get_mctgt_type(struct vm_area_struct *vma,
>   		 * mem_cgroup_move_account() checks the pc is valid or
>   		 * not under LRU exclusion.
>   		 */
> -		if (PageCgroupUsed(pc) && pc->mem_cgroup == mc.from) {
> +		if (pc->mem_cgroup == mc.from) {
>   			ret = MC_TARGET_PAGE;
>   			if (target)
>   				target->page = page;
> @@ -5366,7 +5342,7 @@ static enum mc_target_type get_mctgt_type_thp(struct vm_area_struct *vma,
>   	if (!move_anon())
>   		return ret;
>   	pc = lookup_page_cgroup(page);
> -	if (PageCgroupUsed(pc) && pc->mem_cgroup == mc.from) {
> +	if (pc->mem_cgroup == mc.from) {
>   		ret = MC_TARGET_PAGE;
>   		if (target) {
>   			get_page(page);
> @@ -5810,18 +5786,17 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
>   		return;
>   
>   	pc = lookup_page_cgroup(page);
> +	memcg = pc->mem_cgroup;
>   
>   	/* Readahead page, never charged */
> -	if (!PageCgroupUsed(pc))
> +	if (!memcg)
>   		return;
>   
> -	memcg = pc->mem_cgroup;
> -
>   	oldid = swap_cgroup_record(entry, mem_cgroup_id(memcg));
>   	VM_BUG_ON_PAGE(oldid, page);
>   	mem_cgroup_swap_statistics(memcg, true);
>   
> -	pc->flags = 0;
> +	pc->mem_cgroup = NULL;
>   
>   	if (!mem_cgroup_is_root(memcg))
>   		page_counter_uncharge(&memcg->memory, 1);
> @@ -5895,7 +5870,7 @@ int mem_cgroup_try_charge(struct page *page, struct mm_struct *mm,
>   		 * the page lock, which serializes swap cache removal, which
>   		 * in turn serializes uncharging.
>   		 */
> -		if (PageCgroupUsed(pc))
> +		if (pc->mem_cgroup)
>   			goto out;
>   	}
>   
> @@ -6057,13 +6032,13 @@ static void uncharge_list(struct list_head *page_list)
>   		VM_BUG_ON_PAGE(page_count(page), page);
>   
>   		pc = lookup_page_cgroup(page);
> -		if (!PageCgroupUsed(pc))
> +		if (!pc->mem_cgroup)
>   			continue;
>   
>   		/*
>   		 * Nobody should be changing or seriously looking at
> -		 * pc->mem_cgroup and pc->flags at this point, we have
> -		 * fully exclusive access to the page.
> +		 * pc->mem_cgroup at this point, we have fully
> +		 * exclusive access to the page.
>   		 */
>   
>   		if (memcg != pc->mem_cgroup) {
> @@ -6086,7 +6061,7 @@ static void uncharge_list(struct list_head *page_list)
>   		else
>   			nr_file += nr_pages;
>   
> -		pc->flags = 0;
> +		pc->mem_cgroup = NULL;
>   
>   		pgpgout++;
>   	} while (next != page_list);
> @@ -6112,7 +6087,7 @@ void mem_cgroup_uncharge(struct page *page)
>   
>   	/* Don't touch page->lru of any random page, pre-check: */
>   	pc = lookup_page_cgroup(page);
> -	if (!PageCgroupUsed(pc))
> +	if (!pc->mem_cgroup)
>   		return;
>   
>   	INIT_LIST_HEAD(&page->lru);
> @@ -6148,6 +6123,7 @@ void mem_cgroup_uncharge_list(struct list_head *page_list)
>   void mem_cgroup_migrate(struct page *oldpage, struct page *newpage,
>   			bool lrucare)
>   {
> +	struct mem_cgroup *memcg;
>   	struct page_cgroup *pc;
>   	int isolated;
>   
> @@ -6164,7 +6140,7 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage,
>   
>   	/* Page cache replacement: new page already charged? */
>   	pc = lookup_page_cgroup(newpage);
> -	if (PageCgroupUsed(pc))
> +	if (pc->mem_cgroup)
>   		return;
>   
>   	/*
> @@ -6174,18 +6150,19 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage,
>   	 * reclaim just put back on the LRU but has not released yet.
>   	 */
>   	pc = lookup_page_cgroup(oldpage);
> -	if (!PageCgroupUsed(pc))
> +	memcg = pc->mem_cgroup;
> +	if (!memcg)
>   		return;
>   
>   	if (lrucare)
>   		lock_page_lru(oldpage, &isolated);
>   
> -	pc->flags = 0;
> +	pc->mem_cgroup = NULL;
>   
>   	if (lrucare)
>   		unlock_page_lru(oldpage, isolated);
>   
> -	commit_charge(newpage, pc->mem_cgroup, lrucare);
> +	commit_charge(newpage, memcg, lrucare);
>   }
>   
>   /*
>