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Commit 44b7a8d3 authored by Johannes Weiner's avatar Johannes Weiner Committed by Linus Torvalds
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mm: memcontrol: do not uncharge old page in page cache replacement 



Changing page->mem_cgroup of a live page is tricky and fragile.  In
particular, the memcg writeback code relies on that mapping being stable
and users of mem_cgroup_replace_page() not overlapping with dirtyable
inodes.

Page cache replacement doesn't have to do that, though.  Instead of being
clever and transferring the charge from the old page to the new,
force-charge the new page and leave the old page alone.  A temporary
overcharge won't matter in practice, and the old page is going to be freed
shortly after this anyway.  And this is not performance critical.

Signed-off-by: default avatarJohannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Vladimir Davydov <vdavydov@virtuozzo.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 3e24b19d

mm/memcontrol.c

+15 −11
Original line number Diff line number Diff line
@@ -366,13 +366,6 @@ mem_cgroup_zone_zoneinfo(struct mem_cgroup *memcg, struct zone *zone) 
 *

 * If memcg is bound to a traditional hierarchy, the css of root_mem_cgroup

 * is returned.

 *

 * XXX: The above description of behavior on the default hierarchy isn't

 * strictly true yet as replace_page_cache_page() can modify the

 * association before @page is released even on the default hierarchy;

 * however, the current and planned usages don't mix the the two functions

 * and replace_page_cache_page() will soon be updated to make the invariant

 * actually true.

 */

struct cgroup_subsys_state *mem_cgroup_css_from_page(struct page *page)

{
@@ -5464,7 +5457,8 @@ void mem_cgroup_uncharge_list(struct list_head *page_list) 
void mem_cgroup_replace_page(struct page *oldpage, struct page *newpage)

{

	struct mem_cgroup *memcg;

	int isolated;

	unsigned int nr_pages;

	bool compound;



	VM_BUG_ON_PAGE(!PageLocked(oldpage), oldpage);

	VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
@@ -5484,11 +5478,21 @@ void mem_cgroup_replace_page(struct page *oldpage, struct page *newpage) 
	if (!memcg)

		return;



	lock_page_lru(oldpage, &isolated);

	oldpage->mem_cgroup = NULL;

	unlock_page_lru(oldpage, isolated);

	/* Force-charge the new page. The old one will be freed soon */

	compound = PageTransHuge(newpage);

	nr_pages = compound ? hpage_nr_pages(newpage) : 1;



	page_counter_charge(&memcg->memory, nr_pages);

	if (do_memsw_account())

		page_counter_charge(&memcg->memsw, nr_pages);

	css_get_many(&memcg->css, nr_pages);



	commit_charge(newpage, memcg, true);



	local_irq_disable();

	mem_cgroup_charge_statistics(memcg, newpage, compound, nr_pages);

	memcg_check_events(memcg, newpage);

	local_irq_enable();

}



DEFINE_STATIC_KEY_FALSE(memcg_sockets_enabled_key);