Re: [PATCH v2 2/2] mm: attempt to batch free swap entries for zap_pte_range()

[David Hildenbrand <david@redhat.com>]

>   mm/swapfile.c | 78 +++++++++++++++++++++++++++++++++++++++++++--------
>   1 file changed, 67 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 35cb58373493..25c3f98fa8d5 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -156,6 +156,25 @@ static bool swap_is_has_cache(struct swap_info_struct *si,
>   	return true;
>   }
>   
> +static bool swap_is_last_map(struct swap_info_struct *si,
> +			      unsigned long offset, int nr_pages,
> +			      bool *has_cache)

Please use double tabs for indenting parameters on 2nd line on 
new/changed code:

		unsigned long offset, int nr_pages, bool *has_cache)

Results in less churn when renaming functions and we can frequently 
avoid some lines.

> +{
> +	unsigned char *map = si->swap_map + offset;
> +	unsigned char *map_end = map + nr_pages;
> +	bool cached = false;
> +
> +	do {
> +		if ((*map & ~SWAP_HAS_CACHE) != 1)
> +			return false;
> +		if (*map & SWAP_HAS_CACHE)
> +			cached = true;
> +	} while (++map < map_end);
> +
> +	*has_cache = cached;
> +	return true;
> +}
> +
>   /*
>    * returns number of pages in the folio that backs the swap entry. If positive,
>    * the folio was reclaimed. If negative, the folio was not reclaimed. If 0, no
> @@ -1469,6 +1488,53 @@ static unsigned char __swap_entry_free(struct swap_info_struct *si,
>   	return usage;
>   }
>   
> +static bool __swap_entries_free(struct swap_info_struct *si,
> +				swp_entry_t entry, int nr)

Dito.

> +{
> +	unsigned long offset = swp_offset(entry);
> +	unsigned int type = swp_type(entry);
> +	struct swap_cluster_info *ci;
> +	bool has_cache = false;
> +	unsigned char count;
> +	bool can_batch;
> +	int i;
> +
> +	if (nr <= 1 || swap_count(data_race(si->swap_map[offset])) != 1)
> +		goto fallback;
> +	/* cross into another cluster */
> +	if (nr > SWAPFILE_CLUSTER - offset % SWAPFILE_CLUSTER)
> +		goto fallback;
> +
> +	ci = lock_cluster_or_swap_info(si, offset);
> +	can_batch = swap_is_last_map(si, offset, nr, &has_cache);
> +	if (can_batch) {
> +		for (i = 0; i < nr; i++)
> +			WRITE_ONCE(si->swap_map[offset + i], SWAP_HAS_CACHE);
> +	}
> +	unlock_cluster_or_swap_info(si, ci);
> +
> +	if (!can_batch)
> +		goto fallback;

I'd avoid "can_batch" and just do:

ci = lock_cluster_or_swap_info(si, offset);
if (!swap_is_last_map(si, offset, nr, &has_cache)) {
	unlock_cluster_or_swap_info(si, ci);
	goto fallback;
}
for (i = 0; i < nr; i++)
	WRITE_ONCE(si->swap_map[offset + i], SWAP_HAS_CACHE);
unlock_cluster_or_swap_info(si, ci);

> +	if (!has_cache) {
> +		spin_lock(&si->lock);

I'm no expert on that code, but we might drop the cluster lock the take 
the swap_info lock and then retake the cluster lock. I assume there are 
no races we are worrying about here, right?

> +		swap_entry_range_free(si, entry, nr);
> +		spin_unlock(&si->lock);
> +	}
> +	return has_cache;
> +
> +fallback:
> +	for (i = 0; i  < nr; i++) {

One space too much before the "<".

> +		if (data_race(si->swap_map[offset + i])) {
> +			count = __swap_entry_free(si, swp_entry(type, offset + i));
> +			if (count == SWAP_HAS_CACHE)
> +				has_cache = true;
> +		} else {
> +			WARN_ON_ONCE(1);
> +		}
> +	}
> +	return has_cache;
> +}
> +

-- 
Cheers,

David / dhildenb