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Commit ab936cbc authored by KAMEZAWA Hiroyuki's avatar KAMEZAWA Hiroyuki Committed by Linus Torvalds
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memcg: add mem_cgroup_replace_page_cache() to fix LRU issue 



Commit ef6a3c63 ("mm: add replace_page_cache_page() function") added a
function replace_page_cache_page().  This function replaces a page in the
radix-tree with a new page.  WHen doing this, memory cgroup needs to fix
up the accounting information.  memcg need to check PCG_USED bit etc.

In some(many?) cases, 'newpage' is on LRU before calling
replace_page_cache().  So, memcg's LRU accounting information should be
fixed, too.

This patch adds mem_cgroup_replace_page_cache() and removes the old hooks.
 In that function, old pages will be unaccounted without touching
res_counter and new page will be accounted to the memcg (of old page).
WHen overwriting pc->mem_cgroup of newpage, take zone->lru_lock and avoid
races with LRU handling.

Background:
  replace_page_cache_page() is called by FUSE code in its splice() handling.
  Here, 'newpage' is replacing oldpage but this newpage is not a newly allocated
  page and may be on LRU. LRU mis-accounting will be critical for memory cgroup
  because rmdir() checks the whole LRU is empty and there is no account leak.
  If a page is on the other LRU than it should be, rmdir() will fail.

This bug was added in March 2011, but no bug report yet.  I guess there
are not many people who use memcg and FUSE at the same time with upstream
kernels.

The result of this bug is that admin cannot destroy a memcg because of
account leak.  So, no panic, no deadlock.  And, even if an active cgroup
exist, umount can succseed.  So no problem at shutdown.

Signed-off-by: default avatarKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: default avatarJohannes Weiner <hannes@cmpxchg.org>
Acked-by: default avatarMichal Hocko <mhocko@suse.cz>
Cc: Miklos Szeredi <mszeredi@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 28d82dc1

include/linux/memcontrol.h

+6 −0
Original line number Diff line number Diff line
@@ -122,6 +122,8 @@ struct zone_reclaim_stat* 
mem_cgroup_get_reclaim_stat_from_page(struct page *page);

extern void mem_cgroup_print_oom_info(struct mem_cgroup *memcg,

					struct task_struct *p);

extern void mem_cgroup_replace_page_cache(struct page *oldpage,

					struct page *newpage);



#ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP

extern int do_swap_account;
@@ -369,6 +371,10 @@ static inline 
void mem_cgroup_count_vm_event(struct mm_struct *mm, enum vm_event_item idx)

{

}

static inline void mem_cgroup_replace_page_cache(struct page *oldpage,

				struct page *newpage)

{

}

#endif /* CONFIG_CGROUP_MEM_CONT */



#if !defined(CONFIG_CGROUP_MEM_RES_CTLR) || !defined(CONFIG_DEBUG_VM)

mm/filemap.c

+2 −16
Original line number Diff line number Diff line
@@ -393,24 +393,11 @@ EXPORT_SYMBOL(filemap_write_and_wait_range); 
int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask)

{

	int error;

	struct mem_cgroup *memcg = NULL;



	VM_BUG_ON(!PageLocked(old));

	VM_BUG_ON(!PageLocked(new));

	VM_BUG_ON(new->mapping);



	/*

	 * This is not page migration, but prepare_migration and

	 * end_migration does enough work for charge replacement.

	 *

	 * In the longer term we probably want a specialized function

	 * for moving the charge from old to new in a more efficient

	 * manner.

	 */

	error = mem_cgroup_prepare_migration(old, new, &memcg, gfp_mask);

	if (error)

		return error;



	error = radix_tree_preload(gfp_mask & ~__GFP_HIGHMEM);

	if (!error) {

		struct address_space *mapping = old->mapping;
@@ -432,13 +419,12 @@ int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask) 
		if (PageSwapBacked(new))

			__inc_zone_page_state(new, NR_SHMEM);

		spin_unlock_irq(&mapping->tree_lock);

		/* mem_cgroup codes must not be called under tree_lock */

		mem_cgroup_replace_page_cache(old, new);

		radix_tree_preload_end();

		if (freepage)

			freepage(old);

		page_cache_release(old);

		mem_cgroup_end_migration(memcg, old, new, true);

	} else {

		mem_cgroup_end_migration(memcg, old, new, false);

	}



	return error;

mm/memcontrol.c

+44 −0
Original line number Diff line number Diff line
@@ -3432,6 +3432,50 @@ void mem_cgroup_end_migration(struct mem_cgroup *memcg, 
	cgroup_release_and_wakeup_rmdir(&memcg->css);

}



/*

 * At replace page cache, newpage is not under any memcg but it's on

 * LRU. So, this function doesn't touch res_counter but handles LRU

 * in correct way. Both pages are locked so we cannot race with uncharge.

 */

void mem_cgroup_replace_page_cache(struct page *oldpage,

				  struct page *newpage)

{

	struct mem_cgroup *memcg;

	struct page_cgroup *pc;

	struct zone *zone;

	enum charge_type type = MEM_CGROUP_CHARGE_TYPE_CACHE;

	unsigned long flags;



	if (mem_cgroup_disabled())

		return;



	pc = lookup_page_cgroup(oldpage);

	/* fix accounting on old pages */

	lock_page_cgroup(pc);

	memcg = pc->mem_cgroup;

	mem_cgroup_charge_statistics(memcg, PageCgroupCache(pc), -1);

	ClearPageCgroupUsed(pc);

	unlock_page_cgroup(pc);



	if (PageSwapBacked(oldpage))

		type = MEM_CGROUP_CHARGE_TYPE_SHMEM;



	zone = page_zone(newpage);

	pc = lookup_page_cgroup(newpage);

	/*

	 * Even if newpage->mapping was NULL before starting replacement,

	 * the newpage may be on LRU(or pagevec for LRU) already. We lock

	 * LRU while we overwrite pc->mem_cgroup.

	 */

	spin_lock_irqsave(&zone->lru_lock, flags);

	if (PageLRU(newpage))

		del_page_from_lru_list(zone, newpage, page_lru(newpage));

	__mem_cgroup_commit_charge(memcg, newpage, 1, pc, type);

	if (PageLRU(newpage))

		add_page_to_lru_list(zone, newpage, page_lru(newpage));

	spin_unlock_irqrestore(&zone->lru_lock, flags);

}



#ifdef CONFIG_DEBUG_VM

static struct page_cgroup *lookup_page_cgroup_used(struct page *page)

{