This patchset fixes two kcopyd and dm-snapshot related issues:
1. If kcopyd is not used properly, the kcopyd job slab cache can grow
without limit causing excessive memory usage, which can lead to
user processes being killed by the OOM killer.
2. The kcopyd thread can hog the CPU while processing the submitted
jobs resulting in workqueue stalls and performance degradation.
The root cause for issue (1) is the way dm-snapshot uses kcopyd. In
particular, the lack of an explicit or implicit limit to the maximum
number of in-flight COW jobs.
Issue (2) is caused by:
a. Again, the lack of an upper limit to the number of in-flight kcopyd
jobs created by dm-snapshot.
b. The fact that one can keep inserting new completed jobs through
dm_kcopyd_do_callback() or submitting copy jobs with a source size
of 0, while the kcopyd thread is processing these jobs. This can
lead to the kcopyd thread running completed jobs indefinitely.
Relevant dmesg outputs are provided per patch.
These issues were discovered using the device mapper test suite [1] and
the fixes were also tested with it. The relevant tests used are
documented per patch.
[1] https://github.com/jthornber/device-mapper-test-suite
Nikos Tsironis (2):
dm snapshot: Fix excessive memory usage and workqueue stalls
dm kcopyd: Fix bug causing workqueue stalls
drivers/md/dm-kcopyd.c | 19 ++++++++++++++-----
drivers/md/dm-snap.c | 22 ++++++++++++++++++++++
2 files changed, 36 insertions(+), 5 deletions(-)
--
2.11.0
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