From: Michal Hocko <mhocko@xxxxxxxx>
Subject: mm, page_alloc: remove stop_machine from build_all_zonelists
build_all_zonelists has been (ab)using stop_machine to make sure that
zonelists do not change while somebody is looking at them. This is is
just a gross hack because a) it complicates the context from which we can
call build_all_zonelists (see 3f906ba23689 ("mm/memory-hotplug: switch
locking to a percpu rwsem")) and b) is is not really necessary especially
after "mm, page_alloc: simplify zonelist initialization" and c) it doesn't
really provide the protection it claims (see below).
Updates of the zonelists happen very seldom, basically only when a zone
becomes populated during memory online or when it loses all the memory
during offline. A racing iteration over zonelists could either miss a
zone or try to work on one zone twice. Both of these are something we can
live with occasionally because there will always be at least one zone
visible so we are not likely to fail allocation too easily for example.
Please note that the original stop_machine approach doesn't really provide
a better exclusion because the iteration might be interrupted half way
(unless the whole iteration is preempt disabled which is not the case in
most cases) so the some zones could still be seen twice or a zone missed.
I have run the pathological online/offline of the single memblock in the
movable zone while stressing the same small node with some memory
pressure.
Node 1, zone DMA
pages free 0
min 0
low 0
high 0
spanned 0
present 0
managed 0
protection: (0, 943, 943, 943)
Node 1, zone DMA32
pages free 227310
min 8294
low 10367
high 12440
spanned 262112
present 262112
managed 241436
protection: (0, 0, 0, 0)
Node 1, zone Normal
pages free 0
min 0
low 0
high 0
spanned 0
present 0
managed 0
protection: (0, 0, 0, 1024)
Node 1, zone Movable
pages free 32722
min 85
low 117
high 149
spanned 32768
present 32768
managed 32768
protection: (0, 0, 0, 0)
root@test1:/sys/devices/system/node/node1# while true
do
echo offline > memory34/state
echo online_movable > memory34/state
done
root@test1:/mnt/data/test/linux-3.7-rc5# numactl --preferred=1 make -j4
and it survived without any unexpected behavior. While this is not really
a great testing coverage it should exercise the allocation path quite a
lot.
Link: http://lkml.kernel.org/r/20170721143915.14161-8-mhocko@xxxxxxxxxx
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Joonsoo Kim <js1304@xxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Shaohua Li <shaohua.li@xxxxxxxxx>
Cc: Toshi Kani <toshi.kani@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/page_alloc.c | 9 ++-------
1 file changed, 2 insertions(+), 7 deletions(-)
diff -puN mm/page_alloc.c~mm-page_alloc-remove-stop_machine-from-build_all_zonelists mm/page_alloc.c
--- a/mm/page_alloc.c~mm-page_alloc-remove-stop_machine-from-build_all_zonelists
+++ a/mm/page_alloc.c
@@ -5137,8 +5137,7 @@ static DEFINE_PER_CPU(struct per_cpu_nod
*/
DEFINE_MUTEX(zonelists_mutex);
-/* return values int ....just for stop_machine() */
-static int __build_all_zonelists(void *data)
+static void __build_all_zonelists(void *data)
{
int nid;
int __maybe_unused cpu;
@@ -5174,8 +5173,6 @@ static int __build_all_zonelists(void *d
set_cpu_numa_mem(cpu, local_memory_node(cpu_to_node(cpu)));
#endif
}
-
- return 0;
}
static noinline void __init
@@ -5217,9 +5214,7 @@ void __ref build_all_zonelists(pg_data_t
if (system_state == SYSTEM_BOOTING) {
build_all_zonelists_init();
} else {
- /* we have to stop all cpus to guarantee there is no user
- of zonelist */
- stop_machine_cpuslocked(__build_all_zonelists, pgdat, NULL);
+ __build_all_zonelists(pgdat);
/* cpuset refresh routine should be here */
}
vm_total_pages = nr_free_pagecache_pages();
_
--
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