Re: [PATCH -mm -v3] mm, swap: Sort swap entries before free

[Tim Chen]

> 
> From 7bd903c42749c448ef6acbbdee8dcbc1c5b498b9 Mon Sep 17 00:00:00 2001
> From: Huang Ying <ying.huang@xxxxxxxxx>
> Date: Thu, 23 Feb 2017 13:05:20 +0800
> Subject: [PATCH -v5] mm, swap: Sort swap entries before free
> 
>Â
> ---
> Âmm/swapfile.c | 43 ++++++++++++++++++++++++++++++++++++++-----
> Â1 file changed, 38 insertions(+), 5 deletions(-)
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 71890061f653..10e75f9e8ac1 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -37,6 +37,7 @@
> Â#include <linux/swapfile.h>
> Â#include <linux/export.h>
> Â#include <linux/swap_slots.h>
> +#include <linux/sort.h>
> Â
> Â#include <asm/pgtable.h>
> Â#include <asm/tlbflush.h>
> @@ -1065,20 +1066,52 @@ void swapcache_free(swp_entry_t entry)
> Â	}
> Â}
> Â
> +static int swp_entry_cmp(const void *ent1, const void *ent2)
> +{
> +	const swp_entry_t *e1 = ent1, *e2 = ent2;
> +
> +	return (int)(swp_type(*e1) - swp_type(*e2));
> +}
> +
> Âvoid swapcache_free_entries(swp_entry_t *entries, int n)
> Â{
> Â	struct swap_info_struct *p, *prev;
> -	int i;
> +	int i, m;
> +	swp_entry_t entry;
> +	unsigned int prev_swp_type;

I think it will be clearer to name prev_swp_type as first_swp_type
as this is the swp type of the first entry.

> Â
> Â	if (n <= 0)
> Â		return;
> Â
> Â	prev = NULL;
> Â	p = NULL;
> -	for (i = 0; i < n; ++i) {
> -		p = swap_info_get_cont(entries[i], prev);
> -		if (p)
> -			swap_entry_free(p, entries[i]);
> +	m = 0;
> +	prev_swp_type = swp_type(entries[0]);
> +	for (i = 0; i < n; i++) {
> +		entry = entries[i];
> +		if (likely(swp_type(entry) == prev_swp_type)) {
> +			p = swap_info_get_cont(entry, prev);
> +			if (likely(p))
> +				swap_entry_free(p, entry);
> +			prev = p;
> +		} else if (!m)
> +			m = i;
> +	}
> +	if (p)
> +		spin_unlock(&p->lock);
> +	if (likely(!m))
> +		return;
> +

We could still have prev_swp_type at the first entry after sorting.
and we can avoid an unlock/relock for this case if we do this:

	if (likely(!m)) {
		if (p)
			spin_unlock(&p->lock);
		return;
	}
		
> +	/* Sort swap entries by swap device, so each lock is only taken once. */
> +	sort(entries + m, n - m, sizeof(entries[0]), swp_entry_cmp, NULL);
> +	prev = NULL;

Can eliminate prev=NULL if we adopt the above change.

> +	for (i = m; i < n; i++) {
> +		entry = entries[i];
> +		if (swp_type(entry) == prev_swp_type)
> +			continue;

The if/continue statement seems incorrect. When swp_type(entry) == prev_swp_type
we also need to free entry. ÂThe if/continue statement should be deleted.

Say we have 3 entries with swp_type
1,2,1

We will get prev_swp_type as 1 and free the first entry
and sort the remaining two. ÂThe last entry with
swp_type 1 will not be freed.

> +		p = swap_info_get_cont(entry, prev);
> +		if (likely(p))
> +			swap_entry_free(p, entry);
> Â		prev = p;
> Â	}
> Â	if (p)

Thanks.

Tim