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

[Hugh Dickins <hughd@google.com>]

On Thu, 20 Dec 2018, Andrew Morton wrote:
> 
> Is anyone interested in reviewing this?  Seems somewhat serious. 
> Thanks.

Somewhat serious, but no need to rush.

> 
> From: Yang Shi <yang.shi@linux.alibaba.com>
> 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.

Hugh

> "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.
> 
> Link: http://lkml.kernel.org/r/1541618201-120667-1-git-send-email-yang.shi@linux.alibaba.com
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>  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) {
>  		case PAGEREF_ACTIVATE:
> @@ -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);
> _