Re: [PATCH 4/5] mm/memcontrol: allow to uncharge page without using page->lru field

[Michal Hocko]

On Mon 03-07-17 17:14:14, Jérôme Glisse wrote:
> HMM pages (private or public device pages) are ZONE_DEVICE page and
> thus you can not use page->lru fields of those pages. This patch
> re-arrange the uncharge to allow single page to be uncharge without
> modifying the lru field of the struct page.
> 
> There is no change to memcontrol logic, it is the same as it was
> before this patch.

What is the memcg semantic of the memory? Why is it even charged? AFAIR
this is not a reclaimable memory. If yes how are we going to deal with
memory limits? What should happen if go OOM? Does killing an process
actually help to release that memory? Isn't it pinned by a device?

For the patch itself. It is quite ugly but I haven't spotted anything
obviously wrong with it. It is the memcg semantic with this class of
memory which makes me worried.

> Signed-off-by: Jérôme Glisse <jglisse@xxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
> Cc: cgroups@xxxxxxxxxxxxxxx
> ---
>  mm/memcontrol.c | 168 +++++++++++++++++++++++++++++++-------------------------
>  1 file changed, 92 insertions(+), 76 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 3df3c04d73ab..c709fdceac13 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5509,48 +5509,102 @@ void mem_cgroup_cancel_charge(struct page *page, struct mem_cgroup *memcg,
>  	cancel_charge(memcg, nr_pages);
>  }
>  
> -static void uncharge_batch(struct mem_cgroup *memcg, unsigned long pgpgout,
> -			   unsigned long nr_anon, unsigned long nr_file,
> -			   unsigned long nr_kmem, unsigned long nr_huge,
> -			   unsigned long nr_shmem, struct page *dummy_page)
> +struct uncharge_gather {
> +	struct mem_cgroup *memcg;
> +	unsigned long pgpgout;
> +	unsigned long nr_anon;
> +	unsigned long nr_file;
> +	unsigned long nr_kmem;
> +	unsigned long nr_huge;
> +	unsigned long nr_shmem;
> +	struct page *dummy_page;
> +};
> +
> +static inline void uncharge_gather_clear(struct uncharge_gather *ug)
>  {
> -	unsigned long nr_pages = nr_anon + nr_file + nr_kmem;
> +	memset(ug, 0, sizeof(*ug));
> +}
> +
> +static void uncharge_batch(const struct uncharge_gather *ug)
> +{
> +	unsigned long nr_pages = ug->nr_anon + ug->nr_file + ug->nr_kmem;
>  	unsigned long flags;
>  
> -	if (!mem_cgroup_is_root(memcg)) {
> -		page_counter_uncharge(&memcg->memory, nr_pages);
> +	if (!mem_cgroup_is_root(ug->memcg)) {
> +		page_counter_uncharge(&ug->memcg->memory, nr_pages);
>  		if (do_memsw_account())
> -			page_counter_uncharge(&memcg->memsw, nr_pages);
> -		if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) && nr_kmem)
> -			page_counter_uncharge(&memcg->kmem, nr_kmem);
> -		memcg_oom_recover(memcg);
> +			page_counter_uncharge(&ug->memcg->memsw, nr_pages);
> +		if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) && ug->nr_kmem)
> +			page_counter_uncharge(&ug->memcg->kmem, ug->nr_kmem);
> +		memcg_oom_recover(ug->memcg);
>  	}
>  
>  	local_irq_save(flags);
> -	__this_cpu_sub(memcg->stat->count[MEMCG_RSS], nr_anon);
> -	__this_cpu_sub(memcg->stat->count[MEMCG_CACHE], nr_file);
> -	__this_cpu_sub(memcg->stat->count[MEMCG_RSS_HUGE], nr_huge);
> -	__this_cpu_sub(memcg->stat->count[NR_SHMEM], nr_shmem);
> -	__this_cpu_add(memcg->stat->events[PGPGOUT], pgpgout);
> -	__this_cpu_add(memcg->stat->nr_page_events, nr_pages);
> -	memcg_check_events(memcg, dummy_page);
> +	__this_cpu_sub(ug->memcg->stat->count[MEMCG_RSS], ug->nr_anon);
> +	__this_cpu_sub(ug->memcg->stat->count[MEMCG_CACHE], ug->nr_file);
> +	__this_cpu_sub(ug->memcg->stat->count[MEMCG_RSS_HUGE], ug->nr_huge);
> +	__this_cpu_sub(ug->memcg->stat->count[NR_SHMEM], ug->nr_shmem);
> +	__this_cpu_add(ug->memcg->stat->events[PGPGOUT], ug->pgpgout);
> +	__this_cpu_add(ug->memcg->stat->nr_page_events, nr_pages);
> +	memcg_check_events(ug->memcg, ug->dummy_page);
>  	local_irq_restore(flags);
>  
> -	if (!mem_cgroup_is_root(memcg))
> -		css_put_many(&memcg->css, nr_pages);
> +	if (!mem_cgroup_is_root(ug->memcg))
> +		css_put_many(&ug->memcg->css, nr_pages);
> +}
> +
> +static void uncharge_page(struct page *page, struct uncharge_gather *ug)
> +{
> +	VM_BUG_ON_PAGE(PageLRU(page), page);
> +	VM_BUG_ON_PAGE(!PageHWPoison(page) && page_count(page), page);
> +
> +	if (!page->mem_cgroup)
> +		return;
> +
> +	/*
> +	 * Nobody should be changing or seriously looking at
> +	 * page->mem_cgroup at this point, we have fully
> +	 * exclusive access to the page.
> +	 */
> +
> +	if (ug->memcg != page->mem_cgroup) {
> +		if (ug->memcg) {
> +			uncharge_batch(ug);
> +			uncharge_gather_clear(ug);
> +		}
> +		ug->memcg = page->mem_cgroup;
> +	}
> +
> +	if (!PageKmemcg(page)) {
> +		unsigned int nr_pages = 1;
> +
> +		if (PageTransHuge(page)) {
> +			nr_pages <<= compound_order(page);
> +			ug->nr_huge += nr_pages;
> +		}
> +		if (PageAnon(page))
> +			ug->nr_anon += nr_pages;
> +		else {
> +			ug->nr_file += nr_pages;
> +			if (PageSwapBacked(page))
> +				ug->nr_shmem += nr_pages;
> +		}
> +		ug->pgpgout++;
> +	} else {
> +		ug->nr_kmem += 1 << compound_order(page);
> +		__ClearPageKmemcg(page);
> +	}
> +
> +	ug->dummy_page = page;
> +	page->mem_cgroup = NULL;
>  }
>  
>  static void uncharge_list(struct list_head *page_list)
>  {
> -	struct mem_cgroup *memcg = NULL;
> -	unsigned long nr_shmem = 0;
> -	unsigned long nr_anon = 0;
> -	unsigned long nr_file = 0;
> -	unsigned long nr_huge = 0;
> -	unsigned long nr_kmem = 0;
> -	unsigned long pgpgout = 0;
> +	struct uncharge_gather ug;
>  	struct list_head *next;
> -	struct page *page;
> +
> +	uncharge_gather_clear(&ug);
>  
>  	/*
>  	 * Note that the list can be a single page->lru; hence the
> @@ -5558,57 +5612,16 @@ static void uncharge_list(struct list_head *page_list)
>  	 */
>  	next = page_list->next;
>  	do {
> +		struct page *page;
> +
>  		page = list_entry(next, struct page, lru);
>  		next = page->lru.next;
>  
> -		VM_BUG_ON_PAGE(PageLRU(page), page);
> -		VM_BUG_ON_PAGE(!PageHWPoison(page) && page_count(page), page);
> -
> -		if (!page->mem_cgroup)
> -			continue;
> -
> -		/*
> -		 * Nobody should be changing or seriously looking at
> -		 * page->mem_cgroup at this point, we have fully
> -		 * exclusive access to the page.
> -		 */
> -
> -		if (memcg != page->mem_cgroup) {
> -			if (memcg) {
> -				uncharge_batch(memcg, pgpgout, nr_anon, nr_file,
> -					       nr_kmem, nr_huge, nr_shmem, page);
> -				pgpgout = nr_anon = nr_file = nr_kmem = 0;
> -				nr_huge = nr_shmem = 0;
> -			}
> -			memcg = page->mem_cgroup;
> -		}
> -
> -		if (!PageKmemcg(page)) {
> -			unsigned int nr_pages = 1;
> -
> -			if (PageTransHuge(page)) {
> -				nr_pages <<= compound_order(page);
> -				nr_huge += nr_pages;
> -			}
> -			if (PageAnon(page))
> -				nr_anon += nr_pages;
> -			else {
> -				nr_file += nr_pages;
> -				if (PageSwapBacked(page))
> -					nr_shmem += nr_pages;
> -			}
> -			pgpgout++;
> -		} else {
> -			nr_kmem += 1 << compound_order(page);
> -			__ClearPageKmemcg(page);
> -		}
> -
> -		page->mem_cgroup = NULL;
> +		uncharge_page(page, &ug);
>  	} while (next != page_list);
>  
> -	if (memcg)
> -		uncharge_batch(memcg, pgpgout, nr_anon, nr_file,
> -			       nr_kmem, nr_huge, nr_shmem, page);
> +	if (ug.memcg)
> +		uncharge_batch(&ug);
>  }
>  
>  /**
> @@ -5620,6 +5633,8 @@ static void uncharge_list(struct list_head *page_list)
>   */
>  void mem_cgroup_uncharge(struct page *page)
>  {
> +	struct uncharge_gather ug;
> +
>  	if (mem_cgroup_disabled())
>  		return;
>  
> @@ -5627,8 +5642,9 @@ void mem_cgroup_uncharge(struct page *page)
>  	if (!page->mem_cgroup)
>  		return;
>  
> -	INIT_LIST_HEAD(&page->lru);
> -	uncharge_list(&page->lru);
> +	uncharge_gather_clear(&ug);
> +	uncharge_page(page, &ug);
> +	uncharge_batch(&ug);
>  }
>  
>  /**
> -- 
> 2.13.0

-- 
Michal Hocko
SUSE Labs