Re: [PATCH -mm -v4 05/21] mm, THP, swap: Support PMD swap mapping in free_swap_and_cache()/swap_free()

[Huang\, Ying]

Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> writes:

> On Fri, Jun 22, 2018 at 11:51:35AM +0800, Huang, Ying wrote:
>> +static unsigned char swap_free_cluster(struct swap_info_struct *si,
>> +				       swp_entry_t entry)
> ...
>> +	/* Cluster has been split, free each swap entries in cluster */
>> +	if (!cluster_is_huge(ci)) {
>> +		unlock_cluster(ci);
>> +		for (i = 0; i < SWAPFILE_CLUSTER; i++, entry.val++) {
>> +			if (!__swap_entry_free(si, entry, 1)) {
>> +				free_entries++;
>> +				free_swap_slot(entry);
>> +			}
>> +		}
>
> Is is better on average to use __swap_entry_free_locked instead of
> __swap_entry_free here?  I'm not sure myself, just asking.
>
> As it's written, if the cluster's been split, we always take and drop the
> cluster lock 512 times, but if we don't expect to call free_swap_slot that
> often, then we could just drop and retake the cluster lock inside the innermost
> 'if' against the possibility that free_swap_slot eventually makes us take the
> cluster lock again.

Yes.  This is a good idea.  Thanks for your suggestion!  I will change
this in the next version.

Best Regards,
Huang, Ying

> ...
>> +		return !(free_entries == SWAPFILE_CLUSTER);
>
>                 return free_entries != SWAPFILE_CLUSTER;