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

David Hildenbrand (arm)
2026-04-06T09:21:50Z

On 4/5/26 06:44, Hugh Dickins wrote:
> On Thu, 12 Feb 2026, xu.xin16@zte.com.cn wrote:
>
>> From: xu xin <xu.xin16@zte.com.cn>
>>
>> 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@zte.com.cn/
>>
>> Co-developed-by: Wang Yaxin <wang.yaxin@zte.com.cn>
>> Signed-off-by: Wang Yaxin <wang.yaxin@zte.com.cn>
>> Signed-off-by: xu xin <xu.xin16@zte.com.cn>
>
> This is a very attractive speedup, but I believe it's flawed: in the
> special case when a range has been mremap-moved, when its anon folio
> indexes and anon_vma pgoff correspond to the original user address,
> not to the current user address.

[as discussed in earlier versions of this patch set]

mremap() breaks KSM in the range to be moved.

See prep_move_vma()->ksm_madvise(MADV_UNMERGEABLE)

So I am not sure what you say can trigger.

But I'm just scrolling by, as I'm still busy celebrating Easter :)

--
Cheers,

David