As of v3.11-9444-g3ea67d0 "memcg: add per cgroup writeback pages accounting"
memcg use of __this_cpu_add(counter, -nr_pages) leads to incorrect statistic
values because the negated nr_pages is not sign extended (counter is long,
nr_pages is unsigned int). The memcg fix is __this_cpu_sub(counter, nr_pages).
But that doesn't simply work because __this_cpu_sub(counter, nr_pages) was
implemented as __this_cpu_add(counter, -nr_pages) which suffers the same
problem. Example:
unsigned int delta = 1
preempt_disable()
this_cpu_write(long_counter, 0)
this_cpu_sub(long_counter, delta)
preempt_enable()
Before this change long_counter on a 64 bit machine ends with value 0xffffffff,
rather than 0xffffffffffffffff. This is because this_cpu_sub(pcp, delta) boils
down to:
long_counter = 0 + 0xffffffff
v3.12-rc6 shows that only new memcg code is affected by this problem - the new
mem_cgroup_move_account_page_stat() is the only place where an unsigned
adjustment is used. All other callers (e.g. shrink_dcache_sb) already use a
signed adjustment, so no problems before v3.12. Though I did not audit the
stable kernel trees, so there could be something hiding in there.
Patch 1 creates a test module for percpu operations which demonstrates the
__this_cpu_sub() problems. This patch is independent can be discarded if there
is no interest.
Patch 2 fixes __this_cpu_sub() to work with unsigned adjustments.
Patch 3 uses __this_cpu_sub() in memcg.
An alternative smaller solution is for memcg to use:
__this_cpu_add(counter, -(int)nr_pages)
admitting that __this_cpu_add/sub() doesn't work with unsigned adjustments. But
I felt like fixing the core services to prevent this in the future.
Changes from V1:
- more accurate patch titles, patch logs, and test module description now
referring to per cpu operations rather than per cpu counters.
- move small test code update from patch 2 to patch 1 (where the test is
introduced).
Greg Thelen (3):
percpu: add test module for various percpu operations
percpu: fix this_cpu_sub() subtrahend casting for unsigneds
memcg: use __this_cpu_sub() to dec stats to avoid incorrect subtrahend
casting
arch/x86/include/asm/percpu.h | 3 +-
include/linux/percpu.h | 8 +--
lib/Kconfig.debug | 9 +++
lib/Makefile | 2 +
lib/percpu_test.c | 138 ++++++++++++++++++++++++++++++++++++++++++
mm/memcontrol.c | 2 +-
6 files changed, 156 insertions(+), 6 deletions(-)
create mode 100644 lib/percpu_test.c
--
1.8.4.1
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