linux-kernel - Re: [PATCH 2/2] ksm: Optimize rmap_walk_ksm by passing a suitable address range

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Message-ID: <baef0334-02ea-4732-aa0f-029098879cbd@kernel.org>
Date: Wed, 14 Jan 2026 11:24:56 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: xu.xin16@....com.cn, akpm@...ux-foundation.org
Cc: chengming.zhou@...ux.dev, hughd@...gle.com, wang.yaxin@....com.cn,
 yang.yang29@....com.cn, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH 2/2] ksm: Optimize rmap_walk_ksm by passing a suitable
 address range

On 1/14/26 03:40, xu.xin16@....com.cn wrote:
>>> Solution
>>> ========
>>> In fact, we can significantly improve performance by passing a more precise
>>> range based on the given addr. Since the original pages merged by KSM
>>> correspond to anonymous VMAs, the page offset can be calculated as
>>> pgoff = address >> PAGE_SHIFT. Therefore, we can optimize the call by
>>> defining:
>>>
>>> 	pgoff_start = rmap_item->address >> PAGE_SHIFT;
>>> 	pgoff_end = pgoff_start + folio_nr_pages(folio) - 1;
>>>
>>> Performance
>>> ===========
>>> In our real embedded Linux environment, the measured metrcis were as follows:
>>>
>>> 1) Time_ms: Max time for holding anon_vma lock in a single rmap_walk_ksm.
>>> 2) Nr_iteration_total: The max times of iterations in a loop of anon_vma_interval_tree_foreach
>>> 3) Skip_addr_out_of_range: The max times of skipping due to the first check (vma->vm_start
>>>               and vma->vm_end) in a loop of anon_vma_interval_tree_foreach.
>>> 4) Skip_mm_mismatch: The max times of skipping due to the second check (rmap_item->mm == vma->vm_mm)
>>>               in a loop of anon_vma_interval_tree_foreach.
>>>
>>> The result is as follows:
>>>
>>>                    Time_ms      Nr_iteration_total    Skip_addr_out_of_range   Skip_mm_mismatch
>>> Before patched:  228.65       22169                 22168                    0
>>> After pacthed:   0.396        3                     0                        2
>>
>> Nice improvement.
>>
>> Can you make your reproducer available?
> 
> I'll do my best to try it. The original test data was derived from real business scenarios,
> but it's quite complex. I'll try to simplify this high-latency scenario into a more
> understandable demo as a reproduction program.

Ah, I thought it was some benchmark ran on an embedded environment.

How did you end up measuring these numbers?

> 
>>
>>>
>>> Co-developed-by: Wang Yaxin <wang.yaxin@....com.cn>
>>> Signed-off-by: xu xin <xu.xin16@....com.cn>
>>> ---
>>>    mm/ksm.c | 6 +++++-
>>>    1 file changed, 5 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/ksm.c b/mm/ksm.c
>>> index 335e7151e4a1..0a074ad8e867 100644
>>> --- a/mm/ksm.c
>>> +++ b/mm/ksm.c
>>> @@ -3172,6 +3172,7 @@ void rmap_walk_ksm(struct folio *folio, struct rmap_walk_control *rwc)
>>>    		struct anon_vma_chain *vmac;
>>>    		struct vm_area_struct *vma;
>>>    		unsigned long addr;
>>> +		pgoff_t pgoff_start, pgoff_end;
>>>
>>>    		cond_resched();
>>>    		if (!anon_vma_trylock_read(anon_vma)) {
>>> @@ -3185,8 +3186,11 @@ void rmap_walk_ksm(struct folio *folio, struct rmap_walk_control *rwc)
>>>    		/* Ignore the stable/unstable/sqnr flags */
>>>    		addr = rmap_item->address & PAGE_MASK;
>>>
>>> +		pgoff_start = rmap_item->address >> PAGE_SHIFT;
>>> +		pgoff_end = pgoff_start + folio_nr_pages(folio) - 1;
>>
>> KSM folios are always order-0, so you can keep it simple and hard-code
>> PAGE_SIZE here.
>>
>> You can also initialize both values directly and make them const.
> 
> Yes, I'll do it in v2.

To me, this looks reasonable, but getting some eyes from people more 
familiar with KSM+interval-tree handling would be great. (CCing also Rik)

-- 
Cheers

David