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

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Message-ID: <20260115094158027CMmlQ9-DLqT6FPvCswVli@zte.com.cn>
Date: Thu, 15 Jan 2026 09:41:58 +0800 (CST)
From: <xu.xin16@....com.cn>
To: <david@...nel.org>
Cc: <akpm@...ux-foundation.org>, <chengming.zhou@...ux.dev>,
        <hughd@...gle.com>, <wang.yaxin@....com.cn>, <yang.yang29@....com.cn>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <riel@...riel.com>
Subject: Re: [PATCH 2/2] ksm: Optimize rmap_walk_ksm by passing a suitable address range

> >>> 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?
> 

That was done by inserting livepatch.ko to modify the rmap_walk_ksm().