Re: [PATCH 1/3] mm/mglru: improve readability of isolate_folios()

[Baoquan He]

On 08/20/26 at 12:56pm, Barry Song (Xiaomi) wrote:
> From: Ridong Chen <chenridong@xxxxxxxxxx>
> 
> The for_each_evictable_type() loop in isolate_folios()
> is misleading: it does not actually iterate over each
> evictable type. Instead, get_type_to_scan() selects the
> type to scan, while the iterator `i` merely bounds the
> number of attempts.
> 
> Make the fallback behavior explicit in the code and remove the
> opaque for_each_evictable_type(i, swappiness).
> 
> Signed-off-by: Ridong Chen <chenridong@xxxxxxxxxx>
> Co-developed-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
> Signed-off-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
> ---
>  mm/vmscan.c | 46 ++++++++++++++++++++++++++--------------------
>  1 file changed, 26 insertions(+), 20 deletions(-)

The subject doens't reflect the truth. This patch changes the behaviour,
but not improve readability of isolate_folios() only.

I also noticed the confusion of isolate_folios() implementation, and made
a patch to only change the local variable and added code comment to
explain it in my local branch. Surely refactorying is also good.

+        * Scan at most one type per evictable type (anon/file), starting with
+        * the type get_type_to_scan() picked as statistically colder.
+        *
+        * After a scan:
+        *  - isolated > 0:  got folios, record the type and return.
+        *  - scanned == 0:  the type is empty; fall back to the other type.
+        *  - otherwise:     the type has folios but all were hot (or lost an
+        *                   isolate race); retry the same type rather than
+        *                   switch, so positive_ctrl_err()'s refault
+        *                   statistics stay unbiased.
+        */

While in Ridong's patch, the 3rd case disappeared. It doesn't rescan with
the original type as the old code is doing, but return directly. 


> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index c1404a59523d..d5cc30b667ad 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4833,35 +4833,41 @@ static int get_type_to_scan(struct lruvec *lruvec, int swappiness)
>  	return positive_ctrl_err(&sp, &pv);
>  }
>  
> +static inline bool is_single_type_reclaim(int swappiness)
> +{
> +	return swappiness == MIN_SWAPPINESS ||
> +	       swappiness == SWAPPINESS_ANON_ONLY;
> +}
> +
>  static int isolate_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>  			  struct scan_control *sc, int swappiness,
>  			  struct list_head *list, int *isolated,
>  			  int *isolate_type, int *isolate_scanned)
>  {
> -	int i;
> -	int total_scanned = 0;
> +	bool type_fallback_allowed = !is_single_type_reclaim(swappiness);
>  	int type = get_type_to_scan(lruvec, swappiness);
> +	int total_scanned = 0, scanned, tier;
>  
> -	for_each_evictable_type(i, swappiness) {
> -		int scanned;
> -		int tier = get_tier_idx(lruvec, type);
> +retry:
> +	tier = get_tier_idx(lruvec, type);
> +	scanned = scan_folios(nr_to_scan, lruvec, sc,
> +			      type, tier, list, isolated);
>  
> -		scanned = scan_folios(nr_to_scan, lruvec, sc,
> -				      type, tier, list, isolated);
> +	total_scanned += scanned;
> +	if (*isolated) {
> +		*isolate_type = type;
> +		*isolate_scanned = scanned;
> +		return total_scanned;
> +	}
>  
> -		total_scanned += scanned;
> -		if (*isolated) {
> -			*isolate_type = type;
> -			*isolate_scanned = scanned;
> -			break;
> -		}
> -		/*
> -		 * If scanned > 0 and isolated == 0, avoid falling back to the
> -		 * other type, as this type remains sufficient. Falling back
> -		 * too readily can disrupt the positive_ctrl_err() bias.
> -		 */
> -		if (!scanned)
> -			type = !type;
> +	/*
> +	 * We are running out of the current reclaim type. Fall back to
> +	 * the other type if allowed.
> +	 */
> +	if (!scanned && type_fallback_allowed) {
> +		type = !type;
> +		type_fallback_allowed = false;
> +		goto retry;
>  	}
>  
>  	return total_scanned;
> -- 
> 2.34.1
>