Re: [PATCH 2/2] mm/ksm: add initialization helpers for scan_get_next_rmap_item()

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Thu, Nov 12, 2026 at 08:59:59PM +0800, Xu Xin (vivo) wrote:
> Extract the new scan cycle detection (including draining per-CPU LRU
> caches) and advancing the scan cursor to the first mm_slot into
> scan_begin_new_cycle() and scan_get_first_mm().
>
> This keeps the exact execution order intact while making
> scan_get_next_rmap_item() cleaner and easier to follow.
>
> No functional change intended.
>
> Signed-off-by: Xu Xin (vivo) <xu.xin@linux.dev>

Again, thanks for doing this! LGTM and:

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  mm/ksm.c | 70 +++++++++++++++++++++++++++++++++++++-------------------
>  1 file changed, 46 insertions(+), 24 deletions(-)
>
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 49977bcf04f7..e79e64372e49 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -2640,6 +2640,49 @@ static void ksm_reset_unstable_trees(void)
>  		root_unstable_tree[nid] = RB_ROOT;
>  }
>
> +static bool scan_begin_new_cycle(void)
> +{
> +	/* Check if the cursor points to the head. */
> +	if (ksm_scan.mm_slot != &ksm_mm_head)
> +		return false;
> +
> +	/*
> +	 * A number of pages can hang around indefinitely in per-cpu
> +	 * LRU cache, raised page count preventing write_protect_page
> +	 * from merging them.  Though it doesn't really matter much,
> +	 * it is puzzling to see some stuck in pages_volatile until
> +	 * other activity jostles them out, and they also prevented
> +	 * LTP's KSM test from succeeding deterministically; so drain
> +	 * them here (here rather than on entry to ksm_do_scan(),
> +	 * so we don't IPI too often when pages_to_scan is set low).
> +	 */
> +	lru_add_drain_all();

The sashiko finding is silly, this reorders it after advisor_start_scan() which
calls... ktime_get(), yeah it's zero delta :)

> +	return true;
> +}
> +
> +static struct ksm_mm_slot *scan_get_first_mm(void)
> +{
> +	struct ksm_mm_slot *mm_slot = ksm_scan.mm_slot;
> +	struct mm_slot *slot;
> +
> +	spin_lock(&ksm_mmlist_lock);
> +
> +	slot = list_entry(mm_slot->slot.mm_node.next, struct mm_slot, mm_node);
> +	mm_slot = mm_slot_entry(slot, struct ksm_mm_slot, slot);
> +	ksm_scan.mm_slot = mm_slot;
> +
> +	spin_unlock(&ksm_mmlist_lock);

NIT: optional, but this could be:

	scoped_guard(mutex, &ksm_mmlist_lock) {
		slot = list_entry(mm_slot->slot.mm_node.next, struct mm_slot, mm_node);
		mm_slot = mm_slot_entry(slot, struct ksm_mm_slot, slot);
		ksm_scan.mm_slot = mm_slot;
	}

> +
> +	/*
> +	 * Although we tested list_empty() above, a racing __ksm_exit
> +	 * of the last mm on the list may have removed it since then.
> +	 */
> +	if (mm_slot == &ksm_mm_head)
> +		return NULL;
> +
> +	return mm_slot;
> +}
> +
>  static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
>  {
>  	struct mm_struct *mm;
> @@ -2653,36 +2696,15 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
>  		return NULL;
>
>  	mm_slot = ksm_scan.mm_slot;
> -	if (mm_slot == &ksm_mm_head) {
> +	if (scan_begin_new_cycle()) {
>  		advisor_start_scan();
>  		trace_ksm_start_scan(ksm_scan.seqnr, ksm_rmap_items);
>
> -		/*
> -		 * A number of pages can hang around indefinitely in per-cpu
> -		 * LRU cache, raised page count preventing write_protect_page
> -		 * from merging them.  Though it doesn't really matter much,
> -		 * it is puzzling to see some stuck in pages_volatile until
> -		 * other activity jostles them out, and they also prevented
> -		 * LTP's KSM test from succeeding deterministically; so drain
> -		 * them here (here rather than on entry to ksm_do_scan(),
> -		 * so we don't IPI too often when pages_to_scan is set low).
> -		 */
> -		lru_add_drain_all();
> -
>  		ksm_prune_migrate_nodes();
>  		ksm_reset_unstable_trees();
>
> -		spin_lock(&ksm_mmlist_lock);
> -		slot = list_entry(mm_slot->slot.mm_node.next,
> -				  struct mm_slot, mm_node);
> -		mm_slot = mm_slot_entry(slot, struct ksm_mm_slot, slot);
> -		ksm_scan.mm_slot = mm_slot;
> -		spin_unlock(&ksm_mmlist_lock);
> -		/*
> -		 * Although we tested list_empty() above, a racing __ksm_exit
> -		 * of the last mm on the list may have removed it since then.
> -		 */
> -		if (mm_slot == &ksm_mm_head)
> +		mm_slot = scan_get_first_mm();
> +		if (!mm_slot)
>  			return NULL;
>  next_mm:
>  		ksm_scan.address = 0;
> --
> 2.34.1
>

--
Cheers, Lorenzo