Re: [PATCH] selftests/mm: disable smart scan for ksm_tests

["David Hildenbrand (Arm)" <david@kernel.org>]

On 8/23/26 20:43, Zenghui Yu wrote:
> From: "Zenghui Yu (Huawei)" <zenghui.yu@linux.dev>
> 
> The "KSM NUMA merging" test allocates two identical pages on two NUMA nodes
> and verifies KSM will merge these two pages after 2 scans (see
> ksm_merge_pages()). But when smart scan is enabled, pages that have
> previously not been de-duplicated may get skipped for some scans. Verifying
> KSM behavior after only 2 scans may not be enough.
> 
>  $ ./ksm_tests -N -d
>  TAP version 13
>  1..1
>  pages_shared      : 0
>  pages_sharing     : 0
>  max_page_sharing  : 256
>  full_scans        : 211
>  pages_unshared    : 1
>  pages_volatile    : 2
>  stable_node_chains: 0
>  stable_node_dups  : 0
>  general_profit    : -128
>  ksm_rmap_items 2
>  ksm_zero_pages 0
>  ksm_merging_pages 0
>  ksm_process_profit -128
>  ksm_merge_any: no
>  ksm_mergeable: yes
>  not ok 1 KSM NUMA merging
>  # Totals: pass:0 fail:1 xfail:0 xpass:0 skip:0 error:0

How did you reproduce this? Is this an actual test result?


~/linux/tools/testing/selftests/mm$ cat /sys/kernel/mm/ksm/smart_scan
1
~/linux/tools/testing/selftests/mm$ sudo ./ksm_tests -N -d
TAP version 13
1..1
pages_shared      : 1
pages_sharing     : 1
max_page_sharing  : 256
full_scans        : 1283
pages_unshared    : 0
pages_volatile    : 0
stable_node_chains: 0
stable_node_dups  : 0
general_profit    : 3968
ksm_rmap_items 2
ksm_zero_pages 0
ksm_merging_pages 2
ksm_process_profit 8064
ksm_merge_any: no
ksm_mergeable: yes
ok 1 KSM NUMA merging
# Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0


> 
> This specific test fails because KSM started scanning the second page
> whilst the first page had already been scanned for 16 times

How is that supposed to happen? The sequence we have is:

numa1_map_ptr = numa_alloc_onnode(page_size, first_node);
numa2_map_ptr = numa_alloc_onnode(page_size, second_node);
...
memset(numa1_map_ptr, '*', page_size);
memset(numa2_map_ptr, '*', page_size);
...
if (ksm_merge_pages(merge_type, numa1_map_ptr, page_size, start_time, timeout) ...

KSM smart scan operates on rmap entries. rmap entries are per MM.

-- 
Cheers,

David