Re: [PATCH 10/18] mm: memcontrol: switch to native NR_ANON_MAPPED counter

[Joonsoo Kim]

On Mon, Apr 20, 2020 at 06:11:18PM -0400, Johannes Weiner wrote:
> Memcg maintains a private MEMCG_RSS counter. This divergence from the
> generic VM accounting means unnecessary code overhead, and creates a
> dependency for memcg that page->mapping is set up at the time of
> charging, so that page types can be told apart.
> 
> Convert the generic accounting sites to mod_lruvec_page_state and
> friends to maintain the per-cgroup vmstat counter of
> NR_ANON_MAPPED. We use lock_page_memcg() to stabilize page->mem_cgroup
> during rmap changes, the same way we do for NR_FILE_MAPPED.
> 
> With the previous patch removing MEMCG_CACHE and the private NR_SHMEM
> counter, this patch finally eliminates the need to have page->mapping
> set up at charge time.
> 
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> ---
>  include/linux/memcontrol.h |  3 +--
>  mm/memcontrol.c            | 27 ++++++++--------------
>  mm/rmap.c                  | 47 +++++++++++++++++++++++---------------
>  3 files changed, 39 insertions(+), 38 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index c44aa1ccf553..bfb1d961e346 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -29,8 +29,7 @@ struct kmem_cache;
>  
>  /* Cgroup-specific page state, on top of universal node page state */
>  enum memcg_stat_item {
> -	MEMCG_RSS = NR_VM_NODE_STAT_ITEMS,
> -	MEMCG_RSS_HUGE,
> +	MEMCG_RSS_HUGE = NR_VM_NODE_STAT_ITEMS,
>  	MEMCG_SWAP,
>  	MEMCG_SOCK,
>  	/* XXX: why are these zone and not node counters? */
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 7e77166cf10b..c87178d6219f 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -836,13 +836,6 @@ static void mem_cgroup_charge_statistics(struct mem_cgroup *memcg,
>  					 struct page *page,
>  					 int nr_pages)
>  {
> -	/*
> -	 * Here, RSS means 'mapped anon' and anon's SwapCache. Shmem/tmpfs is
> -	 * counted as CACHE even if it's on ANON LRU.
> -	 */
> -	if (PageAnon(page))
> -		__mod_memcg_state(memcg, MEMCG_RSS, nr_pages);
> -
>  	if (abs(nr_pages) > 1) {
>  		VM_BUG_ON_PAGE(!PageTransHuge(page), page);
>  		__mod_memcg_state(memcg, MEMCG_RSS_HUGE, nr_pages);
> @@ -1384,7 +1377,7 @@ static char *memory_stat_format(struct mem_cgroup *memcg)
>  	 */
>  
>  	seq_buf_printf(&s, "anon %llu\n",
> -		       (u64)memcg_page_state(memcg, MEMCG_RSS) *
> +		       (u64)memcg_page_state(memcg, NR_ANON_MAPPED) *
>  		       PAGE_SIZE);
>  	seq_buf_printf(&s, "file %llu\n",
>  		       (u64)memcg_page_state(memcg, NR_FILE_PAGES) *
> @@ -3298,7 +3291,7 @@ static unsigned long mem_cgroup_usage(struct mem_cgroup *memcg, bool swap)
>  
>  	if (mem_cgroup_is_root(memcg)) {
>  		val = memcg_page_state(memcg, NR_FILE_PAGES) +
> -			memcg_page_state(memcg, MEMCG_RSS);
> +			memcg_page_state(memcg, NR_ANON_MAPPED);
>  		if (swap)
>  			val += memcg_page_state(memcg, MEMCG_SWAP);
>  	} else {
> @@ -3768,7 +3761,7 @@ static int memcg_numa_stat_show(struct seq_file *m, void *v)
>  
>  static const unsigned int memcg1_stats[] = {
>  	NR_FILE_PAGES,
> -	MEMCG_RSS,
> +	NR_ANON_MAPPED,
>  	MEMCG_RSS_HUGE,
>  	NR_SHMEM,
>  	NR_FILE_MAPPED,
> @@ -5395,7 +5388,12 @@ static int mem_cgroup_move_account(struct page *page,
>  
>  	lock_page_memcg(page);
>  
> -	if (!PageAnon(page)) {
> +	if (PageAnon(page)) {
> +		if (page_mapped(page)) {

This page_mapped() check is newly inserted. Could you elaborate more
on why mem_cgroup_charge_statistics() doesn't need this check?

> +			__mod_lruvec_state(from_vec, NR_ANON_MAPPED, -nr_pages);
> +			__mod_lruvec_state(to_vec, NR_ANON_MAPPED, nr_pages);
> +		}
> +	} else {
>  		__mod_lruvec_state(from_vec, NR_FILE_PAGES, -nr_pages);
>  		__mod_lruvec_state(to_vec, NR_FILE_PAGES, nr_pages);
>  
> @@ -6529,7 +6527,6 @@ void mem_cgroup_commit_charge(struct page *page, struct mem_cgroup *memcg,
>  {
>  	unsigned int nr_pages = hpage_nr_pages(page);
>  
> -	VM_BUG_ON_PAGE(!page->mapping, page);
>  	VM_BUG_ON_PAGE(PageLRU(page) && !lrucare, page);
>  
>  	if (mem_cgroup_disabled())
> @@ -6602,8 +6599,6 @@ int mem_cgroup_charge(struct page *page, struct mm_struct *mm, gfp_t gfp_mask,
>  	struct mem_cgroup *memcg;
>  	int ret;
>  
> -	VM_BUG_ON_PAGE(!page->mapping, page);
> -
>  	ret = mem_cgroup_try_charge(page, mm, gfp_mask, &memcg);
>  	if (ret)
>  		return ret;
> @@ -6615,7 +6610,6 @@ struct uncharge_gather {
>  	struct mem_cgroup *memcg;
>  	unsigned long nr_pages;
>  	unsigned long pgpgout;
> -	unsigned long nr_anon;
>  	unsigned long nr_kmem;
>  	unsigned long nr_huge;
>  	struct page *dummy_page;
> @@ -6640,7 +6634,6 @@ static void uncharge_batch(const struct uncharge_gather *ug)
>  	}
>  
>  	local_irq_save(flags);
> -	__mod_memcg_state(ug->memcg, MEMCG_RSS, -ug->nr_anon);
>  	__mod_memcg_state(ug->memcg, MEMCG_RSS_HUGE, -ug->nr_huge);
>  	__count_memcg_events(ug->memcg, PGPGOUT, ug->pgpgout);
>  	__this_cpu_add(ug->memcg->vmstats_percpu->nr_page_events, ug->nr_pages);
> @@ -6682,8 +6675,6 @@ static void uncharge_page(struct page *page, struct uncharge_gather *ug)
>  	if (!PageKmemcg(page)) {
>  		if (PageTransHuge(page))
>  			ug->nr_huge += nr_pages;
> -		if (PageAnon(page))
> -			ug->nr_anon += nr_pages;
>  		ug->pgpgout++;
>  	} else {
>  		ug->nr_kmem += nr_pages;
> diff --git a/mm/rmap.c b/mm/rmap.c
> index f79a206b271a..150513d31efa 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1114,6 +1114,11 @@ void do_page_add_anon_rmap(struct page *page,
>  	bool compound = flags & RMAP_COMPOUND;
>  	bool first;
>  
> +	if (unlikely(PageKsm(page)))
> +		lock_page_memcg(page);
> +	else
> +		VM_BUG_ON_PAGE(!PageLocked(page), page);
> +
>  	if (compound) {
>  		atomic_t *mapcount;
>  		VM_BUG_ON_PAGE(!PageLocked(page), page);
> @@ -1134,12 +1139,13 @@ void do_page_add_anon_rmap(struct page *page,
>  		 */
>  		if (compound)
>  			__inc_node_page_state(page, NR_ANON_THPS);
> -		__mod_node_page_state(page_pgdat(page), NR_ANON_MAPPED, nr);
> +		__mod_lruvec_page_state(page, NR_ANON_MAPPED, nr);
>  	}
> -	if (unlikely(PageKsm(page)))
> -		return;
>  
> -	VM_BUG_ON_PAGE(!PageLocked(page), page);
> +	if (unlikely(PageKsm(page))) {
> +		unlock_page_memcg(page);
> +		return;
> +	}
>  
>  	/* address might be in next vma when migration races vma_adjust */
>  	if (first)
> @@ -1181,7 +1187,7 @@ void page_add_new_anon_rmap(struct page *page,
>  		/* increment count (starts at -1) */
>  		atomic_set(&page->_mapcount, 0);
>  	}
> -	__mod_node_page_state(page_pgdat(page), NR_ANON_MAPPED, nr);
> +	__mod_lruvec_page_state(page, NR_ANON_MAPPED, nr);
>  	__page_set_anon_rmap(page, vma, address, 1);
>  }

memcg isn't setup yet and accounting isn't applied to proper memcg.
Maybe, it would be applied to root memcg. With this change, we don't
need the mapping to commit the charge so switching the order of
page_add_new_anon_rmap() and mem_cgroup_commit_charge() will solve the
issue.

Thanks.