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

[David Hildenbrand (Red Hat)]

On 1/12/26 15:01, 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 Reaon
> ==========
> 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_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?

> Co-developed-by: Wang Yaxin <wang.yaxin@xxxxxxxxxx>
> Signed-off-by: xu xin <xu.xin16@xxxxxxxxxx>
> ---
>   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.

> +
>   		anon_vma_interval_tree_foreach(vmac, &anon_vma->rb_root,
> -					       0, ULONG_MAX) {
> +					       pgoff_start, pgoff_end) {

This is interesting. When we fork() with KSM pages we don't duplicate 
the rmap items. So we rely on this handling here to find all KSM pages 
even in child processes without distinct rmap items.

The important thing is that, whenever we mremap(), we break COW to 
unshare all KSM pages (see prep_move_vma).

So, indeed, I would expect that we only ever have to search at 
rmap->address even in child processes. So makes sense to me.

--
Cheers

David