Re: [patch 13/13] mm: memcontrol: rewrite uncharge API

[Michal Hocko]

On Tue 15-07-14 10:25:45, Michal Hocko wrote:
[...]
> diff --git a/Documentation/cgroups/memcg_test.txt b/Documentation/cgroups/memcg_test.txt
> index bcf750d3cecd..8870b0212150 100644
> --- a/Documentation/cgroups/memcg_test.txt
> +++ b/Documentation/cgroups/memcg_test.txt
[...]
>  6. Shmem(tmpfs) Page Cache
> -	Memcg's charge/uncharge have special handlers of shmem. The best way
> -	to understand shmem's page state transition is to read mm/shmem.c.
> +	The best way to understand shmem's page state transition is to read
> +	mm/shmem.c.

:D

[...]
>  7. Page Migration
> -   	One of the most complicated functions is page-migration-handler.
> -	Memcg has 2 routines. Assume that we are migrating a page's contents
> -	from OLDPAGE to NEWPAGE.
> -
> -	Usual migration logic is..
> -	(a) remove the page from LRU.
> -	(b) allocate NEWPAGE (migration target)
> -	(c) lock by lock_page().
> -	(d) unmap all mappings.
> -	(e-1) If necessary, replace entry in radix-tree.
> -	(e-2) move contents of a page.
> -	(f) map all mappings again.
> -	(g) pushback the page to LRU.
> -	(-) OLDPAGE will be freed.
> -
> -	Before (g), memcg should complete all necessary charge/uncharge to
> -	NEWPAGE/OLDPAGE.
> -
> -	The point is....
> -	- If OLDPAGE is anonymous, all charges will be dropped at (d) because
> -          try_to_unmap() drops all mapcount and the page will not be
> -	  SwapCache.
> -
> -	- If OLDPAGE is SwapCache, charges will be kept at (g) because
> -	  __delete_from_swap_cache() isn't called at (e-1)
> -
> -	- If OLDPAGE is page-cache, charges will be kept at (g) because
> -	  remove_from_swap_cache() isn't called at (e-1)
> -
> -	memcg provides following hooks.
> -
> -	- mem_cgroup_prepare_migration(OLDPAGE)
> -	  Called after (b) to account a charge (usage += PAGE_SIZE) against
> -	  memcg which OLDPAGE belongs to.
> -
> -        - mem_cgroup_end_migration(OLDPAGE, NEWPAGE)
> -	  Called after (f) before (g).
> -	  If OLDPAGE is used, commit OLDPAGE again. If OLDPAGE is already
> -	  charged, a charge by prepare_migration() is automatically canceled.
> -	  If NEWPAGE is used, commit NEWPAGE and uncharge OLDPAGE.
> -
> -	  But zap_pte() (by exit or munmap) can be called while migration,
> -	  we have to check if OLDPAGE/NEWPAGE is a valid page after commit().
> +
> +	mem_cgroup_migrate()

This doesn't tell us anything abouta the page migration. On the other
hand I am not entirely sure the documentation here is very much helpful.
There is some outdated information. I wouldn't be opposed to remove
everything up to "9. Typical Tests." section which should be the primary
target of the file anyway.

>  8. LRU
>          Each memcg has its own private LRU. Now, its handling is under global
[...]
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 290905133078..94fd0b23f3f9 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -382,9 +382,13 @@ static inline int mem_cgroup_swappiness(struct mem_cgroup *mem)
>  }
>  #endif
>  #ifdef CONFIG_MEMCG_SWAP
> -extern void mem_cgroup_uncharge_swap(swp_entry_t ent);
> +extern void mem_cgroup_swapout(struct page *page, swp_entry_t entry);
> +extern void mem_cgroup_uncharge_swap(swp_entry_t entry);

Wouldn't it be nicer to have those two with symmetric names?
mem_cgroup_{un}charge_swap?

[...]
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index fe17420afdc7..e4afdbdda0a7 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
[...]
> @@ -2760,15 +2752,15 @@ static void commit_charge(struct page *page, struct mem_cgroup *memcg,
>  		spin_unlock_irq(&zone->lru_lock);
>  	}
>  
> -	mem_cgroup_charge_statistics(memcg, page, anon, nr_pages);
> -	unlock_page_cgroup(pc);
> -
> +	local_irq_disable();
> +	mem_cgroup_charge_statistics(memcg, page, nr_pages);
>  	/*
>  	 * "charge_statistics" updated event counter. Then, check it.
>  	 * Insert ancestor (and ancestor's ancestors), to softlimit RB-tree.
>  	 * if they exceeds softlimit.
>  	 */
>  	memcg_check_events(memcg, page);
> +	local_irq_enable();

preempt_{enable,disbale} should be sufficient for
mem_cgroup_charge_statistics and memcg_check_events no?
The first one is about per-cpu accounting (and that should be atomic
wrt. IRQ on the same CPU) and the later one uses IRQ safe locks down in
mem_cgroup_update_tree.

Not that it would matter much it is just surprising.

>  }
>  
>  static DEFINE_MUTEX(set_limit_mutex);
[...]
> @@ -3533,20 +3521,23 @@ static int mem_cgroup_move_account(struct page *page,
>  			       nr_pages);
>  	}
>  
> -	mem_cgroup_charge_statistics(from, page, anon, -nr_pages);
> +	/*
> +	 * It is safe to change pc->mem_cgroup here because the page
> +	 * is referenced, charged, and isolated - we can't race with
> +	 * uncharging, charging, migration, or LRU putback.
> +	 */
>  
>  	/* caller should have done css_get */
>  	pc->mem_cgroup = to;
> -	mem_cgroup_charge_statistics(to, page, anon, nr_pages);
>  	move_unlock_mem_cgroup(from, &flags);
>  	ret = 0;
> -unlock:
> -	unlock_page_cgroup(pc);
> -	/*
> -	 * check events
> -	 */
> +
> +	local_irq_disable();
> +	mem_cgroup_charge_statistics(to, page, nr_pages);
>  	memcg_check_events(to, page);
> +	mem_cgroup_charge_statistics(from, page, -nr_pages);
>  	memcg_check_events(from, page);
> +	local_irq_enable();
>  out:
>  	return ret;
>  }

Same here.

[...]
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 63f0cd559999..9da3cf84d30a 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -780,11 +780,14 @@ static int move_to_new_page(struct page *newpage, struct page *page,
>  		rc = fallback_migrate_page(mapping, newpage, page, mode);
>  
>  	if (rc != MIGRATEPAGE_SUCCESS) {
> -		newpage->mapping = NULL;
> +		if (!PageAnon(newpage))
> +			newpage->mapping = NULL;

OK, I am probably washed out from looking into this for too long but I
cannot figure why have you done this...

>  	} else {
> +		mem_cgroup_migrate(page, newpage, false);
>  		if (remap_swapcache)
>  			remove_migration_ptes(page, newpage);
> -		page->mapping = NULL;
> +		if (!PageAnon(page))
> +			page->mapping = NULL;
>  	}
>  
>  	unlock_page(newpage);

[...]

The semantic is much cleaner now. I have to digest details about the
patch because it is really huge. But nothing really jumped at me during
the review (except for few minor things mentioned here and one mentioned
in other email regarding USED flag).

Good work! 
-- 
Michal Hocko
SUSE Labs
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