Re: [PATCH resend] ksm: add merge_only_zero_pages to merge only empty anonymous pages

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

On 11/9/26 03:02, Xu Xin (vivo) wrote:
> On Sat, Nov 07, 2026 at 04:13:40PM +0800, Jinjiang Tu wrote:
>>
>> 在 2026/11/6 11:48, xu.xin@linux.dev 写道:
>>> From: "Xu Xin (vivo)" <xu.xin@linux.dev>
>>>
>>>    KSM side-channel exposure is desired. Many handset vendors may prefer
>>>    this knob, which can be used as a default fallback policy.
>>>
>>> - Not suitable for server and virtualization workloads containing many
>>>    non-zero duplicate pages, where vanilla KSM provides better memory
>>>    savings.
>>
>> It’s a coincidence that I’m also working on similar optimizations.
>> In our scenario, we have a MySQL instance with over 700 GB of anonymous
>> memory. Most of this memory belongs to the buffer pool, which caches
>> data from storage; these pages cannot be merged. Of the remaining memory,
>> around 15 GB is consumed by hashtables, which contain many zero-filled
>> pages that are eligible for merging to reduce memory footprint.
>>
>> When we enabled KSM for the entire MySQL instance, ksmd created rmap_item
>> metadata for each candidate page.The massive metadata overhead wiped out
>> any gains from page merging, and even resulted in a negative benefit.
>>
>> For this reason, I’m trying to make ksmd merge only zero pages. The goal
>> is to have ksmd skip scanning non-zero pages entirely, thereby avoiding
>> the rmap_item metadata cost introduced by scanning non-mergeable pages.
> 
> Skip generating ksm_rmap_item for non-zero pages ? That dose make sense to me.

Yes, we should avoid any such memory overhead in that case.

-- 
Cheers,

David