On 01/29/2013 10:24 PM, Michal Hocko wrote:

On Mon 28-01-13 18:54:40, Jeff Liu wrote:

quoted

As we don't account the swap stat number for the root_mem_cgroup anymore,
here we can just return an invalid CSS ID if there is no non-root memcg
is alive.  Also, introduce memsw_accounting_users to track the number of
active non-root memcgs.

Signed-off-by: Jie Liu <redacted>
CC: Glauber Costa <redacted>
CC: Michal Hocko <redacted>
CC: Kamezawa Hiroyuki <redacted>
CC: Johannes Weiner <hannes@cmpxchg.org>
CC: Mel Gorman <mgorman@suse.de>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Sha Zhengju <redacted>

---
 mm/page_cgroup.c |   16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/mm/page_cgroup.c b/mm/page_cgroup.c
index c945254..189fbf5 100644
--- a/mm/page_cgroup.c
+++ b/mm/page_cgroup.c

@@ -336,6 +336,8 @@ struct swap_cgroup {
 };
 #define SC_PER_PAGE	(PAGE_SIZE/sizeof(struct swap_cgroup))
 
+static atomic_t memsw_accounting_users = ATOMIC_INIT(0);

Nobody manipulates whith this number. I suspect the next patch will do
but it is generally better to have also users of the introduced counters
in the same patch. For one thing this patch would introduce a regression
because no pages would be accounted at this stage (for example during
git bisect).

Ah, I'll revise it accordingly.

Thanks,
-Jeff

quoted

+
 /*
  * SwapCgroup implements "lookup" and "exchange" operations.
  * In typical usage, this swap_cgroup is accessed via memcg's charge/uncharge

@@ -389,6 +391,9 @@ static struct swap_cgroup *lookup_swap_cgroup(swp_entry_t ent,
 	struct page *mappage;
 	struct swap_cgroup *sc;
 
+	if (!atomic_read(&memsw_accounting_users))
+		return NULL;
+
 	ctrl = &swap_cgroup_ctrl[swp_type(ent)];
 	if (ctrlp)
 		*ctrlp = ctrl;

@@ -416,6 +421,8 @@ unsigned short swap_cgroup_cmpxchg(swp_entry_t ent,
 	unsigned short retval;
 
 	sc = lookup_swap_cgroup(ent, &ctrl);
+	if (!sc)
+		return 0;
 
 	spin_lock_irqsave(&ctrl->lock, flags);
 	retval = sc->id;

@@ -443,6 +450,8 @@ unsigned short swap_cgroup_record(swp_entry_t ent, unsigned short id)
 	unsigned long flags;
 
 	sc = lookup_swap_cgroup(ent, &ctrl);
+	if (!sc)
+		return 0;
 
 	spin_lock_irqsave(&ctrl->lock, flags);
 	old = sc->id;

@@ -460,7 +469,9 @@ unsigned short swap_cgroup_record(swp_entry_t ent, unsigned short id)
  */
 unsigned short lookup_swap_cgroup_id(swp_entry_t ent)
 {
-	return lookup_swap_cgroup(ent, NULL)->id;
+	struct swap_cgroup *sc = lookup_swap_cgroup(ent, NULL);
+
+	return sc ? sc->id : 0;
 }
 
 int swap_cgroup_swapon(int type, unsigned long max_pages)

@@ -471,6 +482,9 @@ int swap_cgroup_swapon(int type, unsigned long max_pages)
 	if (!do_swap_account)
 		return 0;
 
+	if (!atomic_read(&memsw_accounting_users))
+		return 0;
+
 	length = DIV_ROUND_UP(max_pages, SC_PER_PAGE);
 
 	ctrl = &swap_cgroup_ctrl[type];

-- 
1.7.9.5

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