[PATCH 0/2] mm/memcontrol: fix reclaim bugs in mem_cgroup_iter

[PATCH 0/2] mm/memcontrol: fix reclaim bugs in mem_cgroup_iter

[Sean Christopherson]

This patch set contains two bug fixes for mem_cgroup_iter().  The bugs
were found by code inspection and were confirmed via synthetic testing
that forcefully setup the failing conditions.

Bug #1 is a race condition where mem_cgroup_iter() incorrectly returns
the same memcg to multiple threads reclaiming from the same root, zone,
priority and generation.  mem_cgroup_iter() doesn't check the result of
cmpxchg(iter->pos...) when setting the new pos, and so fails to detect
that it will return the same memcg as the thread that successfully set
iter->position.  If multiple threads read the same iter->position value,
then they will call css_next_descendant_pre() with the same css and will
compute the same memcg (unless they see different versions of the tree
due to an RCU update).

Bug #2 is also a race condition of sorts, with the same setup conditions
as bug #1.  If a reclaimer's initial call to mem_cgroup_iter() triggers
a restart of the hierarchy walk, i.e. css_next_descendant_pre() returns
NULL and prev == NULL, mem_cgroup_iter() fails to increment iter->gen...
even though it has started a new walk of the hierarchy.  This technically
isn't a bug for the thread that triggered the restart as it's reasonable
for that thread to perform a full walk of the tree, but other threads
in the current reclaim generation will incorrectly continue to walk the
tree since iter->generation won't be updated until one of the reclaimers
reaches the end of the hierarchy a second time.

The two patches can be applied independently, but I included them in a
single series as the fix for bug #1 can theoretically exacerbate bug #2,
and bug #2 is likely more serious as it results in a duplicate walk of
the entire tree as opposed to a duplicate reclaim of a single memcg.


Sean Christopherson (2):
  mm/memcontrol: check cmpxchg(iter->pos...) result in mem_cgroup_iter()
  mm/memcontrol: inc reclaim gen if restarting walk in mem_cgroup_iter()

 mm/memcontrol.c | 56 +++++++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 47 insertions(+), 9 deletions(-)


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[PATCH 1/2] mm/memcontrol: check cmpxchg(iter->pos...) result in mem_cgroup_iter()

[Sean Christopherson]

Check the return value of cmpxchg when updating iter->position in
mem_cgroup_iter().  If cmpxchg failed, i.e. a different thread won
the race to update iter->position, then restart the entire flow of
reading, processing and updating iter->position.  Simply ensuring
that there aren't multiple writes to iter->position doesn't avoid
redundant reclaims of a memcg, as competing threads will compute
the same memcg given the same iter->position.

The cmpxchg will only fail if a different thread saw the same value
of iter->position, meaning it called css_next_descendant_pre() with
the same css and therefore computed the same memcg (ignoring the
corner case where the threads see different versions of the tree).

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
 mm/memcontrol.c | 25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 16c556a..6a7ca3c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -758,6 +758,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 
 	rcu_read_lock();
 
+start:
 	if (reclaim) {
 		struct mem_cgroup_per_node *mz;
 
@@ -818,11 +819,27 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 
 	if (reclaim) {
 		/*
-		 * The position could have already been updated by a competing
-		 * thread, so check that the value hasn't changed since we read
-		 * it to avoid reclaiming from the same cgroup twice.
+		 * Competing reclaim threads may attempt to consume the same
+		 * iter->position, check that the value hasn't changed since
+		 * we read it to avoid reclaiming from the same cgroup twice.
+		 * Note that just avoiding multiple writes to iter->position
+		 * does not prevent redundant reclaims to memcg.  Given the
+		 * same input css on competing threads, the css returned by
+		 * css_next_descendant_pre will also be the same (unless the
+		 * tree itself changes).  So, if a different thread read the
+		 * same iter->position, then it also computed the same memcg.
+		 * If we lost the race, put our css references and restart
+		 * the entire process of reading and updating iter->position.
 		 */
-		(void)cmpxchg(&iter->position, pos, memcg);
+		if (cmpxchg(&iter->position, pos, memcg) != pos) {
+			if (memcg && memcg != root)
+				css_put(&memcg->css);
+			if (pos)
+				css_put(&pos->css);
+			css = NULL;
+			memcg = NULL;
+			goto start;
+		}
 
 		if (pos)
 			css_put(&pos->css);
-- 
2.7.4

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[PATCH 2/2] mm/memcontrol: inc reclaim gen if restarting walk in mem_cgroup_iter()

[Sean Christopherson]

Increment iter->generation if a reclaimer reaches the end of the tree,
even if it restarts the hierarchy walk instead of returning NULL, i.e.
this is the reclaimer's initial call to mem_cgroup_iter().  If we don't
increment the generation, other threads that are part of the current
reclaim generation will incorrectly continue to walk the tree since
iter->generation won't be updated until one of the reclaimers reaches
the end of the hierarchy a second time.

Move the put_css(&pos->css) call below the iter->generation update
to minimize the window where a thread can see a stale generation but
consume an updated position, as iter->generation and iter->position
are not updated atomically.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
 mm/memcontrol.c | 31 ++++++++++++++++++++++++++-----
 1 file changed, 26 insertions(+), 5 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6a7ca3c..b858245 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -740,6 +740,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 	struct cgroup_subsys_state *css = NULL;
 	struct mem_cgroup *memcg = NULL;
 	struct mem_cgroup *pos = NULL;
+	bool inc_gen = false;
 
 	if (mem_cgroup_disabled())
 		return NULL;
@@ -791,6 +792,14 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 		css = css_next_descendant_pre(css, &root->css);
 		if (!css) {
 			/*
+			 * Increment the generation as the next call to
+			 * css_next_descendant_pre will restart at root.
+			 * Do not update iter->generation directly as we
+			 * should only do so if we update iter->position.
+			 */
+			inc_gen = true;
+
+			/*
 			 * Reclaimers share the hierarchy walk, and a
 			 * new one might jump in right at the end of
 			 * the hierarchy - make sure they see at least
@@ -838,16 +847,28 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 				css_put(&pos->css);
 			css = NULL;
 			memcg = NULL;
+			inc_gen = false;
 			goto start;
 		}
 
-		if (pos)
-			css_put(&pos->css);
-
-		if (!memcg)
+		/*
+		 * Update iter->generation asap to minimize the window where
+		 * a different thread compares against a stale generation but
+		 * consumes an updated position.
+		 */
+		if (inc_gen)
 			iter->generation++;
-		else if (!prev)
+
+		/*
+		 * Initialize the reclaimer's generation after the potential
+		 * update to iter->generation; if we restarted the hierarchy
+		 * walk then we are part of the new generation.
+		 */
+		if (!prev)
 			reclaim->generation = iter->generation;
+
+		if (pos)
+			css_put(&pos->css);
 	}
 
 out_unlock:
-- 
2.7.4

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Re: [PATCH 0/2] mm/memcontrol: fix reclaim bugs in mem_cgroup_iter

[Michal Hocko]

On Fri 28-04-17 14:55:45, Sean Christopherson wrote:
> This patch set contains two bug fixes for mem_cgroup_iter().  The bugs
> were found by code inspection and were confirmed via synthetic testing
> that forcefully setup the failing conditions.

I assume that you added some artificial sleeps to make those races more
probable, right? Or did you manage to hit those issue solely from the
userspace? I will have a look at those patches. It has been some time
since I've had it cached. It is pretty subtle code so I would like to
understand the urgency before I dive into this further.
-- 
Michal Hocko
SUSE Labs

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RE: [PATCH 0/2] mm/memcontrol: fix reclaim bugs in mem_cgroup_iter

[Christopherson, Sean J]

Michal Hocko <mhocko@kernel.org> wrote:
> On Fri 28-04-17 14:55:45, Sean Christopherson wrote:
> > This patch set contains two bug fixes for mem_cgroup_iter().  The bugs
> > were found by code inspection and were confirmed via synthetic testing
> > that forcefully setup the failing conditions.
> 
> I assume that you added some artificial sleeps to make those races more
> probable, right? Or did you manage to hit those issue solely from the
> userspace? I will have a look at those patches. It has been some time
> since I've had it cached. It is pretty subtle code so I would like to
> understand the urgency before I dive into this further.
> -- 
> Michal Hocko
> SUSE Labs

The code to prove the bugs is completely artificial, it's basically a
unit test for mem_cgroup_iter() that uses a thread barrier to all but
guarantee two threads will call mem_cgroup_iter() simultaneously.  I
haven't even attempted to hit this via the actual userspace flow.

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