Re: [PATCH v3 2/2] ksm: Optimize rmap_walk_ksm by passing a suitable address range

[David Hildenbrand (Arm)]

On 2/12/26 12:30, xu.xin16@xxxxxxxxxx wrote:
> From: xu xin <xu.xin16@xxxxxxxxxx>
>
> Problem
> =======
> When available memory is extremely tight, causing KSM pages to be swapped
> out, or when there is significant memory fragmentation and THP triggers
> memory compaction, the system will invoke the rmap_walk_ksm function to
> perform reverse mapping. However, we observed that this function becomes
> particularly time-consuming when a large number of VMAs (e.g., 20,000)
> share the same anon_vma. Through debug trace analysis, we found that most
> of the latency occurs within anon_vma_interval_tree_foreach, leading to an
> excessively long hold time on the anon_vma lock (even reaching 500ms or
> more), which in turn causes upper-layer applications (waiting for the
> anon_vma lock) to be blocked for extended periods.
>
> Root Cause
> ==========
> Further investigation revealed that 99.9% of iterations inside the
> anon_vma_interval_tree_foreach loop are skipped due to the first check
> "if (addr < vma->vm_start || addr >= vma->vm_end)), indicating that a large
> number of loop iterations are ineffective. This inefficiency arises because
> the pgoff_start and pgoff_end parameters passed to
> anon_vma_interval_tree_foreach span the entire address space from 0 to
> ULONG_MAX, resulting in very poor loop efficiency.
>
> 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 = rmap_item->address >> PAGE_SHIFT;
>
> 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:  228.65       22169                 22168                    0
> After :   0.396        3                     0                       2
>
> The referenced reproducer of rmap_walk_ksm can be found at:
> https://lore.kernel.org/all/20260206151424734QIyWL_pA-1QeJPbJlUxsO@xxxxxxxxxx/
>
> Co-developed-by: Wang Yaxin <wang.yaxin@xxxxxxxxxx>
> Signed-off-by: Wang Yaxin <wang.yaxin@xxxxxxxxxx>
> Signed-off-by: xu xin <xu.xin16@xxxxxxxxxx>
> ---

Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

--
Cheers,

David