[PATCH mmotm,next] mm: fix memcg-less page reclaim

[PATCH mmotm,next] mm: fix memcg-less page reclaim

[Hugh Dickins]

Now that everybody loves memcg, configures it on, and would not dream
of booting with cgroup_disable=memory, it can pass unnoticed for weeks
that memcg-less page reclaim is completely broken.

mmotm's "memcg: enhance memcg iterator to support predicates" replaces
__shrink_zone()'s "do { } while (memcg);" loop by a "while (memcg) {}"
loop: which is nicer for memcg, but does nothing for !CONFIG_MEMCG or
cgroup_disable=memory.  Page reclaim hangs, making no progress.

Adding mem_cgroup_disabled() and once++ test there is ugly.  Ideally,
even a !CONFIG_MEMCG build might in future have a stub root_mem_cgroup,
which would get around this: but that's not so at present.

However, it appears that nothing actually dereferences the memcg pointer
in the mem_cgroup_disabled() case, here or anywhere else that case can
reach mem_cgroup_iter() (mem_cgroup_iter_break() is not called in
global reclaim).

So, simply pass back an ordinarily-oopsing non-NULL address the first
time, and we shall hear about it if I'm wrong.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
By all means fold in to
memcg-enhance-memcg-iterator-to-support-predicates.patch

 include/linux/memcontrol.h |    3 ++-
 mm/memcontrol.c            |    6 ++++--
 2 files changed, 6 insertions(+), 3 deletions(-)

--- 3.11-rc5-mm1/include/linux/memcontrol.h	2013-08-15 18:10:50.504539510 -0700
+++ linux/include/linux/memcontrol.h	2013-08-18 12:30:58.116460318 -0700
@@ -370,7 +370,8 @@ mem_cgroup_iter_cond(struct mem_cgroup *
 		struct mem_cgroup_reclaim_cookie *reclaim,
 		mem_cgroup_iter_filter cond)
 {
-	return NULL;
+	/* first call must return non-NULL, second return NULL */
+	return (struct mem_cgroup *)(unsigned long)!prev;
 }
 
 static inline struct mem_cgroup *
--- 3.11-rc5-mm1/mm/memcontrol.c	2013-08-15 18:10:50.720539516 -0700
+++ linux/mm/memcontrol.c	2013-08-18 12:29:15.352460818 -0700
@@ -1086,8 +1086,10 @@ struct mem_cgroup *mem_cgroup_iter_cond(
 	struct mem_cgroup *memcg = NULL;
 	struct mem_cgroup *last_visited = NULL;
 
-	if (mem_cgroup_disabled())
-		return NULL;
+	if (mem_cgroup_disabled()) {
+		/* first call must return non-NULL, second return NULL */
+		return (struct mem_cgroup *)(unsigned long)!prev;
+	}
 
 	if (!root)
 		root = root_mem_cgroup;

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Re: [PATCH mmotm,next] mm: fix memcg-less page reclaim

[Michal Hocko]

On Sun 18-08-13 23:05:25, Hugh Dickins wrote:
> Now that everybody loves memcg, configures it on, and would not dream
> of booting with cgroup_disable=memory, it can pass unnoticed for weeks
> that memcg-less page reclaim is completely broken.
> 
> mmotm's "memcg: enhance memcg iterator to support predicates" replaces
> __shrink_zone()'s "do { } while (memcg);" loop by a "while (memcg) {}"
> loop: which is nicer for memcg, but does nothing for !CONFIG_MEMCG or
> cgroup_disable=memory.  Page reclaim hangs, making no progress.

Ouch. Very well spotted, Hugh! I have totally missed this...
 
> Adding mem_cgroup_disabled() and once++ test there is ugly.  Ideally,
> even a !CONFIG_MEMCG build might in future have a stub root_mem_cgroup,
> which would get around this: but that's not so at present.
> 
> However, it appears that nothing actually dereferences the memcg pointer
> in the mem_cgroup_disabled() case, here or anywhere else that case can
> reach mem_cgroup_iter() (mem_cgroup_iter_break() is not called in
> global reclaim).
> 
> So, simply pass back an ordinarily-oopsing non-NULL address the first
> time, and we shall hear about it if I'm wrong.

This is a bit tricky but it seems like the easiest way for now. I will
look at the fake root cgroup for !CONFIG_MEMCG.

> Signed-off-by: Hugh Dickins <hughd@google.com>

Acked-by: Michal Hocko <mhocko@suse.cz>

Thanks!

> ---
> By all means fold in to
> memcg-enhance-memcg-iterator-to-support-predicates.patch
> 
>  include/linux/memcontrol.h |    3 ++-
>  mm/memcontrol.c            |    6 ++++--
>  2 files changed, 6 insertions(+), 3 deletions(-)
> 
> --- 3.11-rc5-mm1/include/linux/memcontrol.h	2013-08-15 18:10:50.504539510 -0700
> +++ linux/include/linux/memcontrol.h	2013-08-18 12:30:58.116460318 -0700
> @@ -370,7 +370,8 @@ mem_cgroup_iter_cond(struct mem_cgroup *
>  		struct mem_cgroup_reclaim_cookie *reclaim,
>  		mem_cgroup_iter_filter cond)
>  {
> -	return NULL;
> +	/* first call must return non-NULL, second return NULL */
> +	return (struct mem_cgroup *)(unsigned long)!prev;
>  }
>  
>  static inline struct mem_cgroup *
> --- 3.11-rc5-mm1/mm/memcontrol.c	2013-08-15 18:10:50.720539516 -0700
> +++ linux/mm/memcontrol.c	2013-08-18 12:29:15.352460818 -0700
> @@ -1086,8 +1086,10 @@ struct mem_cgroup *mem_cgroup_iter_cond(
>  	struct mem_cgroup *memcg = NULL;
>  	struct mem_cgroup *last_visited = NULL;
>  
> -	if (mem_cgroup_disabled())
> -		return NULL;
> +	if (mem_cgroup_disabled()) {
> +		/* first call must return non-NULL, second return NULL */
> +		return (struct mem_cgroup *)(unsigned long)!prev;
> +	}
>  
>  	if (!root)
>  		root = root_mem_cgroup;

-- 
Michal Hocko
SUSE Labs

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Re: [PATCH mmotm,next] mm: fix memcg-less page reclaim

[Michal Hocko]

[Let's CC Johannes, Kamezewa and Kosaki]

On Mon 19-08-13 09:44:07, Michal Hocko wrote:
> On Sun 18-08-13 23:05:25, Hugh Dickins wrote:
[...]
> > Adding mem_cgroup_disabled() and once++ test there is ugly.  Ideally,
> > even a !CONFIG_MEMCG build might in future have a stub root_mem_cgroup,
> > which would get around this: but that's not so at present.
> > 
> > However, it appears that nothing actually dereferences the memcg pointer
> > in the mem_cgroup_disabled() case, here or anywhere else that case can
> > reach mem_cgroup_iter() (mem_cgroup_iter_break() is not called in
> > global reclaim).
> > 
> > So, simply pass back an ordinarily-oopsing non-NULL address the first
> > time, and we shall hear about it if I'm wrong.
> 
> This is a bit tricky but it seems like the easiest way for now. I will
> look at the fake root cgroup for !CONFIG_MEMCG.

OK, the following builds for both CONFIG_MEMCG enabled and disabled and
should work with cgroup_disable=memory as well as we are allocating
root_mem_cgroup for disabled case as well AFAICS.

It looks less scary than I expected. I haven't tested it yet but if you
think that it looks promising I will send a full patch with changelog.
---

Re: [PATCH mmotm,next] mm: fix memcg-less page reclaim

[Johannes Weiner]

On Mon, Aug 19, 2013 at 11:51:36AM +0200, Michal Hocko wrote:
> [Let's CC Johannes, Kamezewa and Kosaki]
> 
> On Mon 19-08-13 09:44:07, Michal Hocko wrote:
> > On Sun 18-08-13 23:05:25, Hugh Dickins wrote:
> [...]
> > > Adding mem_cgroup_disabled() and once++ test there is ugly.  Ideally,
> > > even a !CONFIG_MEMCG build might in future have a stub root_mem_cgroup,
> > > which would get around this: but that's not so at present.
> > > 
> > > However, it appears that nothing actually dereferences the memcg pointer
> > > in the mem_cgroup_disabled() case, here or anywhere else that case can
> > > reach mem_cgroup_iter() (mem_cgroup_iter_break() is not called in
> > > global reclaim).
> > > 
> > > So, simply pass back an ordinarily-oopsing non-NULL address the first
> > > time, and we shall hear about it if I'm wrong.
> > 
> > This is a bit tricky but it seems like the easiest way for now. I will
> > look at the fake root cgroup for !CONFIG_MEMCG.
> 
> OK, the following builds for both CONFIG_MEMCG enabled and disabled and
> should work with cgroup_disable=memory as well as we are allocating
> root_mem_cgroup for disabled case as well AFAICS.
> 
> It looks less scary than I expected. I haven't tested it yet but if you
> think that it looks promising I will send a full patch with changelog.
> ---
> >From 954085db1837874f94e0249e74b5ae1b49dcb9f8 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.cz>
> Date: Mon, 19 Aug 2013 10:51:07 +0200
> Subject: [PATCH] memcg: add a fake root_mem_cgroup for !CONFIG_MEMCG
> 
> TODO proper changelog
> 
> Signed-off-by: Michal Hocko <mhocko@suse.cz>
> ---
>  include/linux/memcontrol.h |    8 ++++++--
>  mm/Makefile                |    3 +++
>  mm/fake_root_memcg.c       |   14 ++++++++++++++
>  mm/memcontrol.c            |   17 +++++++++++++----
>  mm/vmscan.c                |    8 ++++----
>  5 files changed, 40 insertions(+), 10 deletions(-)
>  create mode 100644 mm/fake_root_memcg.c
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 8a9ed4d..1d795a8 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -25,6 +25,8 @@
>  #include <linux/jump_label.h>
>  
>  struct mem_cgroup;
> +struct mem_cgroup *get_root_mem_cgroup(void);
> +
>  struct page_cgroup;
>  struct page;
>  struct mm_struct;
> @@ -370,7 +372,9 @@ mem_cgroup_iter_cond(struct mem_cgroup *root,
>  		struct mem_cgroup_reclaim_cookie *reclaim,
>  		mem_cgroup_iter_filter cond)
>  {
> -	return NULL;
> +	if (prev)
> +		return NULL;
> +	return root;
>  }
>  
>  static inline struct mem_cgroup *
> @@ -378,7 +382,7 @@ mem_cgroup_iter(struct mem_cgroup *root,
>  		struct mem_cgroup *prev,
>  		struct mem_cgroup_reclaim_cookie *reclaim)
>  {
> -	return NULL;
> +	return mem_cgroup_iter_cond(root, prev, reclaim, NULL);
>  }
>  
>  static inline void mem_cgroup_iter_break(struct mem_cgroup *root,
> diff --git a/mm/Makefile b/mm/Makefile
> index 305d10a..fadc984 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -52,6 +52,9 @@ obj-$(CONFIG_MIGRATION) += migrate.o
>  obj-$(CONFIG_QUICKLIST) += quicklist.o
>  obj-$(CONFIG_TRANSPARENT_HUGEPAGE) += huge_memory.o
>  obj-$(CONFIG_MEMCG) += memcontrol.o page_cgroup.o vmpressure.o
> +ifndef CONFIG_MEMCG
> +	obj-y 		+= fake_root_memcg.o
> +endif
>  obj-$(CONFIG_CGROUP_HUGETLB) += hugetlb_cgroup.o
>  obj-$(CONFIG_MEMORY_FAILURE) += memory-failure.o
>  obj-$(CONFIG_HWPOISON_INJECT) += hwpoison-inject.o
> diff --git a/mm/fake_root_memcg.c b/mm/fake_root_memcg.c
> new file mode 100644
> index 0000000..e98bd1e
> --- /dev/null
> +++ b/mm/fake_root_memcg.c
> @@ -0,0 +1,14 @@
> +#include <linux/memcontrol.h>
> +
> +/* Make a type placeholder for root_mem_cgroup. */
> +struct mem_cgroup {};
> +
> +/*
> + * This is a fake root_mem_cgroup which will be used as a placeholder
> + * for !CONFIG_MEMCG.
> + */
> +struct mem_cgroup root_mem_cgroup;
> +struct mem_cgroup *get_root_mem_cgroup(void)
> +{
> +	return &root_mem_cgroup;
> +}

Yuck, enough with these hacks already.  This is why memcontrol.c is an
unmaintainable heap of garbage.  And now it's metastasizing into other
things in mm, including all that unreadable softlimit stuff in
mm/vmscan.c.  This has to stop.

If you want to make root_mem_cgroup always available, which is not a
bad idea IMO and hch suggested during the lruvec patches, then make it
properly in mecontrol.c and move the lruvec from struct zone in there.
Then we can actually get rid of indirections and special cases, not
add even more.

Or make the reclaim iterators return lruvecs.  They are so convoluted
at this point that it would actually be an improvement to
maintainability if they were separate code.

Maybe both.

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Re: [PATCH mmotm,next] mm: fix memcg-less page reclaim

[Hugh Dickins]

On Mon, 19 Aug 2013, Michal Hocko wrote:
> [Let's CC Johannes, Kamezewa and Kosaki]
> 
> On Mon 19-08-13 09:44:07, Michal Hocko wrote:
> > On Sun 18-08-13 23:05:25, Hugh Dickins wrote:
> [...]
> > > Adding mem_cgroup_disabled() and once++ test there is ugly.  Ideally,
> > > even a !CONFIG_MEMCG build might in future have a stub root_mem_cgroup,
> > > which would get around this: but that's not so at present.
> > > 
> > > However, it appears that nothing actually dereferences the memcg pointer
> > > in the mem_cgroup_disabled() case, here or anywhere else that case can
> > > reach mem_cgroup_iter() (mem_cgroup_iter_break() is not called in
> > > global reclaim).
> > > 
> > > So, simply pass back an ordinarily-oopsing non-NULL address the first
> > > time, and we shall hear about it if I'm wrong.
> > 
> > This is a bit tricky but it seems like the easiest way for now. I will
> > look at the fake root cgroup for !CONFIG_MEMCG.
> 
> OK, the following builds for both CONFIG_MEMCG enabled and disabled and
> should work with cgroup_disable=memory as well as we are allocating
> root_mem_cgroup for disabled case as well AFAICS.
> 
> It looks less scary than I expected. I haven't tested it yet but if you
> think that it looks promising I will send a full patch with changelog.

Sorry, I think this is merely more complicated and wasteful than
the two-liner I sent.  And I'd be surprised if it's handling the
cgroup_disable=memory case correctly.

I do imagine that one day a root_mem_cgroup for all may provide useful
simplifications - avoiding many of the mem_cgroup_disabled() tests
scattered around, for example.  But it's not obvious what fields that
ought to have initialized; and it's not obvious how many of the
skip-doing-this-on-root-memcg decisions should be re-evaluated
in going that way.  It's not something to rush into.

I don't see any point in introducing it now, solely for the
mem_cgroup_iter_cond() loop: that's better served by my patch.

Sorry if my comment misled you into doing this: I meant that your iter
loop would work naturally if everyone had a non-NULL root_mem_cgroup,
and maybe in future that will be the case, but not now.

Hugh

> ---
> From 954085db1837874f94e0249e74b5ae1b49dcb9f8 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.cz>
> Date: Mon, 19 Aug 2013 10:51:07 +0200
> Subject: [PATCH] memcg: add a fake root_mem_cgroup for !CONFIG_MEMCG
> 
> TODO proper changelog
> 
> Signed-off-by: Michal Hocko <mhocko@suse.cz>
> ---
>  include/linux/memcontrol.h |    8 ++++++--
>  mm/Makefile                |    3 +++
>  mm/fake_root_memcg.c       |   14 ++++++++++++++
>  mm/memcontrol.c            |   17 +++++++++++++----
>  mm/vmscan.c                |    8 ++++----
>  5 files changed, 40 insertions(+), 10 deletions(-)
>  create mode 100644 mm/fake_root_memcg.c
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 8a9ed4d..1d795a8 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -25,6 +25,8 @@
>  #include <linux/jump_label.h>
>  
>  struct mem_cgroup;
> +struct mem_cgroup *get_root_mem_cgroup(void);
> +
>  struct page_cgroup;
>  struct page;
>  struct mm_struct;
> @@ -370,7 +372,9 @@ mem_cgroup_iter_cond(struct mem_cgroup *root,
>  		struct mem_cgroup_reclaim_cookie *reclaim,
>  		mem_cgroup_iter_filter cond)
>  {
> -	return NULL;
> +	if (prev)
> +		return NULL;
> +	return root;
>  }
>  
>  static inline struct mem_cgroup *
> @@ -378,7 +382,7 @@ mem_cgroup_iter(struct mem_cgroup *root,
>  		struct mem_cgroup *prev,
>  		struct mem_cgroup_reclaim_cookie *reclaim)
>  {
> -	return NULL;
> +	return mem_cgroup_iter_cond(root, prev, reclaim, NULL);
>  }
>  
>  static inline void mem_cgroup_iter_break(struct mem_cgroup *root,
> diff --git a/mm/Makefile b/mm/Makefile
> index 305d10a..fadc984 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -52,6 +52,9 @@ obj-$(CONFIG_MIGRATION) += migrate.o
>  obj-$(CONFIG_QUICKLIST) += quicklist.o
>  obj-$(CONFIG_TRANSPARENT_HUGEPAGE) += huge_memory.o
>  obj-$(CONFIG_MEMCG) += memcontrol.o page_cgroup.o vmpressure.o
> +ifndef CONFIG_MEMCG
> +	obj-y 		+= fake_root_memcg.o
> +endif
>  obj-$(CONFIG_CGROUP_HUGETLB) += hugetlb_cgroup.o
>  obj-$(CONFIG_MEMORY_FAILURE) += memory-failure.o
>  obj-$(CONFIG_HWPOISON_INJECT) += hwpoison-inject.o
> diff --git a/mm/fake_root_memcg.c b/mm/fake_root_memcg.c
> new file mode 100644
> index 0000000..e98bd1e
> --- /dev/null
> +++ b/mm/fake_root_memcg.c
> @@ -0,0 +1,14 @@
> +#include <linux/memcontrol.h>
> +
> +/* Make a type placeholder for root_mem_cgroup. */
> +struct mem_cgroup {};
> +
> +/*
> + * This is a fake root_mem_cgroup which will be used as a placeholder
> + * for !CONFIG_MEMCG.
> + */
> +struct mem_cgroup root_mem_cgroup;
> +struct mem_cgroup *get_root_mem_cgroup(void)
> +{
> +	return &root_mem_cgroup;
> +}
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index b73988a..69ed11b 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -68,6 +68,11 @@ EXPORT_SYMBOL(mem_cgroup_subsys);
>  #define MEM_CGROUP_RECLAIM_RETRIES	5
>  static struct mem_cgroup *root_mem_cgroup __read_mostly;
>  
> +struct mem_cgroup *get_root_mem_cgroup(void)
> +{
> +	return root_mem_cgroup;

I expect that returns NULL when booted with cgroup_disable=memory.

> +}
> +
>  #ifdef CONFIG_MEMCG_SWAP
>  /* Turned on only when memory cgroup is enabled && really_do_swap_account = 1 */
>  int do_swap_account __read_mostly;
> @@ -1109,8 +1114,12 @@ struct mem_cgroup *mem_cgroup_iter_cond(struct mem_cgroup *root,
>  	struct mem_cgroup *memcg = NULL;
>  	struct mem_cgroup *last_visited = NULL;
>  
> -	if (mem_cgroup_disabled())
> -		return NULL;
> +	VM_BUG_ON(!root);
> +	if (mem_cgroup_disabled()) {
> +		if (prev)
> +			return NULL;
> +		return root;
> +	}
>  
>  	if (!root)
>  		root = root_mem_cgroup;
> @@ -1198,9 +1207,9 @@ void mem_cgroup_iter_break(struct mem_cgroup *root,
>  	     iter = mem_cgroup_iter(root, iter, NULL))
>  
>  #define for_each_mem_cgroup(iter)			\
> -	for (iter = mem_cgroup_iter(NULL, NULL, NULL);	\
> +	for (iter = mem_cgroup_iter(root_mem_cgroup, NULL, NULL);	\
>  	     iter != NULL;				\
> -	     iter = mem_cgroup_iter(NULL, iter, NULL))
> +	     iter = mem_cgroup_iter(root_mem_cgroup, iter, NULL))
>  
>  void __mem_cgroup_count_vm_event(struct mm_struct *mm, enum vm_event_item idx)
>  {
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a3bf7fd..d10b44a 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2596,7 +2596,7 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
>  		.may_swap = 1,
>  		.order = order,
>  		.priority = DEF_PRIORITY,
> -		.target_mem_cgroup = NULL,
> +		.target_mem_cgroup = get_root_mem_cgroup(),
>  		.nodemask = nodemask,
>  	};
>  	struct shrink_control shrink = {
> @@ -2714,7 +2714,7 @@ static void age_active_anon(struct zone *zone, struct scan_control *sc)
>  	if (!total_swap_pages)
>  		return;
>  
> -	memcg = mem_cgroup_iter(NULL, NULL, NULL);
> +	memcg = mem_cgroup_iter(get_root_mem_cgroup(), NULL, NULL);
>  	do {
>  		struct lruvec *lruvec = mem_cgroup_zone_lruvec(zone, memcg);
>  
> @@ -2722,7 +2722,7 @@ static void age_active_anon(struct zone *zone, struct scan_control *sc)
>  			shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
>  					   sc, LRU_ACTIVE_ANON);
>  
> -		memcg = mem_cgroup_iter(NULL, memcg, NULL);
> +		memcg = mem_cgroup_iter(get_root_mem_cgroup(), memcg, NULL);
>  	} while (memcg);
>  }
>  
> @@ -2949,7 +2949,7 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>  		.may_swap = 1,
>  		.may_writepage = !laptop_mode,
>  		.order = order,
> -		.target_mem_cgroup = NULL,
> +		.target_mem_cgroup = get_root_mem_cgroup(),
>  	};
>  	count_vm_event(PAGEOUTRUN);
>  
> -- 
> 1.7.10.4
> 
> -- 
> Michal Hocko
> SUSE Labs

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Re: [PATCH mmotm,next] mm: fix memcg-less page reclaim

[Michal Hocko]

On Mon 19-08-13 12:28:31, Hugh Dickins wrote:
[...]
> I don't see any point in introducing it now, solely for the
> mem_cgroup_iter_cond() loop: that's better served by my patch.

OK. Fair enough. I thougt it would be at least clear if we accidently
added a code which would require touching the non-existing memcg for
!MEMCG case because that wouldn't even compile with this patch.
-- 
Michal Hocko
SUSE Labs

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Re: [PATCH mmotm,next] mm: fix memcg-less page reclaim

[Michal Hocko]

On Mon 19-08-13 11:45:38, Johannes Weiner wrote:
[...]
> If you want to make root_mem_cgroup always available, which is not a
> bad idea IMO and hch suggested during the lruvec patches, then make it
> properly in mecontrol.c and move the lruvec from struct zone in there.
> Then we can actually get rid of indirections and special cases, not
> add even more.
> 
> Or make the reclaim iterators return lruvecs.  They are so convoluted
> at this point that it would actually be an improvement to
> maintainability if they were separate code.
> 
> Maybe both.

Will think about it.

-- 
Michal Hocko
SUSE Labs

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