Re: [PATCH cgroup/for-4.3-fixes] cgroup, writeback: don't enable cgroup writeback on traditional hierarchies

From: Jens Axboe
Date: Thu Sep 24 2015 - 12:19:01 EST

On 09/23/2015 03:07 PM, Tejun Heo wrote:
inode_cgwb_enabled() gates cgroup writeback support. If it returns
true, each inode is attached to the corresponding memory domain which
gets mapped to io domain. It currently only tests whether the
filesystem and bdi support cgroup writeback; however, cgroup writeback
support doesn't work on traditional hierarchies and thus it should
also test whether memcg and iocg are on the default hierarchy.

This caused traditional hierarchy setups to hit the cgroup writeback
path inadvertently and ended up creating separate writeback domains
for each memcg and mapping them all to the root iocg uncovering a
couple issues in the cgroup writeback path.

cgroup writeback was never meant to be enabled on traditional
hierarchies. Make inode_cgwb_enabled() test whether both memcg and
iocg are on the default hierarchy.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reported-by: Artem Bityutskiy <dedekind1@xxxxxxxxx>
Reported-by: Dexuan Cui <decui@xxxxxxxxxxxxx>

So, this should make the regression go away. It doesn't fix the
underlying bugs but they shouldn't get triggered by people not
experimenting with cgroup.

I'm gonna keep digging the underlying issues but this should make the
regressions go away. If it's okay, I think it'd be better to route
this through cgroup/for-4.3-fixes as it's gonna cause a conflict with
for-4.4 branch and handling the merge there is easier.


include/linux/backing-dev.h | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)

I'll ack this since it works around both the corruption issue and the performance regression, so we can avoid having to revert parts of this. And I know you'll keep hunting and get the real issue fixed in the mean time.

Acked-by: Jens Axboe <axboe@xxxxxx>

Jens Axboe

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