Re: [PATCH 1/2] mm: vmscan: skip KSM page in direct reclaim if priority is low

From: Yang Shi
Date: Fri Dec 21 2018 - 01:34:59 EST

On 12/20/18 10:04 PM, Hugh Dickins wrote:
On Thu, 20 Dec 2018, Andrew Morton wrote:
Is anyone interested in reviewing this? Seems somewhat serious.
Somewhat serious, but no need to rush.

From: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
Subject: mm: vmscan: skip KSM page in direct reclaim if priority is low

When running a stress test, we occasionally run into the below hang issue:
Artificial load presumably.

INFO: task ksmd:205 blocked for more than 360 seconds.
Tainted: G E 4.9.128-001.ali3000_nightly_20180925_264.alios7.x86_64 #1
4.9-stable does not contain Andrea's 4.13 commit 2c653d0ee2ae
("ksm: introduce ksm_max_page_sharing per page deduplication limit").

The patch below is more economical than Andrea's, but I don't think
a second workaround should be added, unless Andrea's is shown to be
insufficient, even with its ksm_max_page_sharing tuned down to suit.

Yang, please try to reproduce on upstream, or backport Andrea's to
4.9-stable - thanks.

I believe Andrea's commit could workaround this problem too by limiting the number of sharing pages.

However, IMHO, even though we just have a few hundred pages share one KSM page, it still sounds not worth reclaiming it in direct reclaim in low priority. According to Andrea's commit log, it still takes a few msec to walk the rmap for 256 shared pages.



"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
ksmd D 0 205 2 0x00000000
ffff882fa00418c0 0000000000000000 ffff882fa4b10000 ffff882fbf059d00
ffff882fa5bc1800 ffffc900190c7c28 ffffffff81725e58 ffffffff810777c0
00ffc900190c7c88 ffff882fbf059d00 ffffffff8138cc09 ffff882fa4b10000
Call Trace:
[<ffffffff81725e58>] ? __schedule+0x258/0x720
[<ffffffff810777c0>] ? do_flush_tlb_all+0x30/0x30
[<ffffffff8138cc09>] ? free_cpumask_var+0x9/0x10
[<ffffffff81726356>] schedule+0x36/0x80
[<ffffffff81729916>] schedule_timeout+0x206/0x4b0
[<ffffffff81077d0f>] ? native_flush_tlb_others+0x11f/0x180
[<ffffffff8110ca40>] ? ktime_get+0x40/0xb0
[<ffffffff81725b6a>] io_schedule_timeout+0xda/0x170
[<ffffffff81726c50>] ? bit_wait+0x60/0x60
[<ffffffff81726c6b>] bit_wait_io+0x1b/0x60
[<ffffffff81726759>] __wait_on_bit_lock+0x59/0xc0
[<ffffffff811aff76>] __lock_page+0x86/0xa0
[<ffffffff810d53e0>] ? wake_atomic_t_function+0x60/0x60
[<ffffffff8121a269>] ksm_scan_thread+0xeb9/0x1430
[<ffffffff810d5340>] ? prepare_to_wait_event+0x100/0x100
[<ffffffff812193b0>] ? try_to_merge_with_ksm_page+0x850/0x850
[<ffffffff810ac226>] kthread+0xe6/0x100
[<ffffffff810ac140>] ? kthread_park+0x60/0x60
[<ffffffff8172b196>] ret_from_fork+0x46/0x60

ksmd found a suitable KSM page on the stable tree and is trying to lock
it. But it is locked by the direct reclaim path which is walking the
page's rmap to get the number of referenced PTEs.

The KSM page rmap walk needs to iterate all rmap_items of the page and all
rmap anon_vmas of each rmap_item. So it may take (# rmap_item * #
children processes) loops. This number of loops might be very large in
the worst case, and may take a long time.

Typically, direct reclaim will not intend to reclaim too many pages, and
it is latency sensitive. So it is not worth doing the long ksm page rmap
walk to reclaim just one page.

Skip KSM pages in direct reclaim if the reclaim priority is low, but still
try to reclaim KSM pages with high priority.

Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>

mm/vmscan.c | 23 +++++++++++++++++++++--
1 file changed, 21 insertions(+), 2 deletions(-)

--- a/mm/vmscan.c~mm-vmscan-skip-ksm-page-in-direct-reclaim-if-priority-is-low
+++ a/mm/vmscan.c
@@ -1260,8 +1260,17 @@ static unsigned long shrink_page_list(st
- if (!force_reclaim)
- references = page_check_references(page, sc);
+ if (!force_reclaim) {
+ /*
+ * Don't try to reclaim KSM page in direct reclaim if
+ * the priority is not high enough.
+ */
+ if (PageKsm(page) && !current_is_kswapd() &&
+ sc->priority > (DEF_PRIORITY - 2))
+ references = PAGEREF_KEEP;
+ else
+ references = page_check_references(page, sc);
+ }
switch (references) {
@@ -2136,6 +2145,16 @@ static void shrink_active_list(unsigned
+ /*
+ * Skip KSM page in direct reclaim if priority is not
+ * high enough.
+ */
+ if (PageKsm(page) && !current_is_kswapd() &&
+ sc->priority > (DEF_PRIORITY - 2)) {
+ putback_lru_page(page);
+ continue;
+ }
if (page_referenced(page, 0, sc->target_mem_cgroup,
&vm_flags)) {
nr_rotated += hpage_nr_pages(page);