Re: [PATCH v2 1/2] mm/ksm: align smart scan skips with the scan number

[David Hildenbrand (Arm)]

On 11/6/26 10:45, Xu Xin (vivo) wrote:
> On Thu, Nov 05, 2026 at 04:36:14AM -0500, Qiliang Yuan wrote:
>> Smart scan skips pages that KSM has failed to de-duplicate for a while.
>> Once a page is three scans old, its rmap_item counts down its own number
>> of scans to skip, 1, 2, 4 and then 8 as it ages, so an old page is looked
>> at every ninth scan.
>>
>> The unstable tree is rebuilt on every scan, so two identical pages that
>> are not in the stable tree yet can only be merged in a scan that looks
>> at both. Each countdown runs from when its own page became mergeable, so
>> two pages that became mergeable at different times end up on different
>> phases and, once both are old, may never be looked at in the same scan
>> again. They then stay unmerged for as long as they live. Making a page
>> mergeable, waiting for 0 to 20 full scans before doing the same with an
>> identical page, 20 times for each wait, and checking the pair after
>> another 64 full scans shows this, on two boots per kernel:
>>
>>                                 before            after
>>   pairs merged (of 420)         329 / 317         420 / 420
>>   64M of unique pages skipped   89.0% / 89.1%     87.5% / 87.5%
>>   ksmd CPU time per scan        6.25 / 6.25 ms    6.88 / 7.03 ms
>>
>> Pick the skipped scans by the global scan number instead, with periods
>> of 2, 4 and 8 scans. The periods being powers of two, every old page is
>> looked at on each scan that is a multiple of 8, whatever its age, so any
>> two pages meet at least every eighth scan. The longest period goes from
>> nine scans to eight, which costs 10-12% more ksmd time on memory that
>> never merges.
>>
>> Fixes: 5e924ff54d08 ("mm/ksm: add "smart" page scanning mode")
>> Signed-off-by: Qiliang Yuan <odys.yuan@xxxxxxxxx>
>> ---
>>  mm/ksm.c | 24 ++++++++++--------------
>>  1 file changed, 10 insertions(+), 14 deletions(-)
> 
> Brilliant design.
> By using the global ksm_scan.seqnr as a unified time base, this patch eliminates
> the out-of-sync problem of the original per-page skip counters.
> 
> It guarantees that any two pages will be processed in the same scan at least once
> every 8 scans — the least common multiple of all scan periods — which completely
> resolves the permanent de-duplication failure.
> 
> Admittedly, this comes at the cost of shrinking the longest scan interval from 9
> to 8 scans, raising the scan frequency for aged pages by roughly 12.5%. But I
> believe this is a very reasonable trade-off for correctness. So
> 
> Reviewed-by: Xu Xin (vivo) <xu.xin@xxxxxxxxx>
> 
> 
> Besides, can we add a extra description about new smart_scan algorithm that we
> missed in Documentation/mm/ksm.rst?

Agreed, that would be nice. Thanks!

-- 
Cheers,

David