"Problems" with the current code:
1. there is a lack of synchronization in setting ->high and ->batch in
percpu_pagelist_fraction_sysctl_handler()
2. stop_machine() in zone_pcp_update() is unnecissary.
3. zone_pcp_update() does not consider the case where percpu_pagelist_fraction is non-zero
To fix:
1. add memory barriers, a safe ->batch value, an update side mutex when
updating ->high and ->batch, and use ACCESS_ONCE() for ->batch users that
expect a stable value.
2. avoid draining pages in zone_pcp_update(), rely upon the memory barriers added to fix #1
3. factor out quite a few functions, and then call the appropriate one.
Note that it results in a change to the behavior of zone_pcp_update(), which is
used by memory_hotplug. I'm rather certain that I've diserned (and preserved)
the essential behavior (changing ->high and ->batch), and only eliminated
unneeded actions (draining the per cpu pages), but this may not be the case.
Further note that the draining of pages that previously took place in
zone_pcp_update() occured after repeated draining when attempting to offline a
page, and after the offline has "succeeded". It appears that the draining was
added to zone_pcp_update() to avoid refactoring setup_pageset() into 2
funtions.
--
Since v2: https://lkml.org/lkml/2013/4/9/718
- note ACCESS_ONCE() in fix #1 above.
- consolidate ->batch & ->high update protocol into a single funtion (Gilad).
- add missing ACCESS_ONCE() on ->batch
Since v1: https://lkml.org/lkml/2013/4/5/444
- instead of using on_each_cpu(), use memory barriers (Gilad) and an update side mutex.
- add "Problem" #3 above, and fix.
- rename function to match naming style of similar function
- move unrelated comment
--
Cody P Schafer (11):
mm/page_alloc: factor out setting of pcp->high and pcp->batch.
mm/page_alloc: prevent concurrent updaters of pcp ->batch and ->high
mm/page_alloc: insert memory barriers to allow async update of pcp
batch and high
mm/page_alloc: protect pcp->batch accesses with ACCESS_ONCE
mm/page_alloc: convert zone_pcp_update() to rely on memory barriers
instead of stop_machine()
mm/page_alloc: when handling percpu_pagelist_fraction, don't unneedly
recalulate high
mm/page_alloc: factor setup_pageset() into pageset_init() and
pageset_set_batch()
mm/page_alloc: relocate comment to be directly above code it refers
to.
mm/page_alloc: factor zone_pageset_init() out of setup_zone_pageset()
mm/page_alloc: in zone_pcp_update(), uze zone_pageset_init()
mm/page_alloc: rename setup_pagelist_highmark() to match naming of
pageset_set_batch()
mm/page_alloc.c | 151 +++++++++++++++++++++++++++++++++-----------------------
1 file changed, 90 insertions(+), 61 deletions(-)
--
1.8.2.2
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