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    SubjectRe: [PATCH cgroup/for-4.3-fixes] cgroup, writeback: don't enable cgroup writeback on traditional hierarchies
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    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@kernel.org>
    > Reported-by: Artem Bityutskiy <dedekind1@gmail.com>
    > Reported-by: Dexuan Cui <decui@microsoft.com>
    > Link: http://lkml.kernel.org/g/1443012552.19983.209.camel@gmail.com
    > Link: http://lkml.kernel.org/g/f30d4a6aa8a546ff88f73021d026a453@SIXPR30MB031.064d.mgd.msft.net
    > ---
    > Hello,
    >
    > 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.
    >
    > Thanks.
    >
    > 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@fb.com>

    --
    Jens Axboe



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