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From: "Huang\, Ying" <ying.huang@intel.com>
Subject: Re: [PATCH -mm -v4 05/21] mm, THP, swap: Support PMD swap mapping in free_swap_and_cache()/swap_free()
References: <20180622035151.6676-1-ying.huang@intel.com>
	<20180622035151.6676-6-ying.huang@intel.com>
	<20180705183318.je4gd32awgh2tnb5@ca-dmjordan1.us.oracle.com>
Date: Fri, 06 Jul 2018 20:49:05 +0800
In-Reply-To: <20180705183318.je4gd32awgh2tnb5@ca-dmjordan1.us.oracle.com>
	(Daniel Jordan's message of "Thu, 5 Jul 2018 11:33:18 -0700")
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To: Daniel Jordan <daniel.m.jordan@oracle.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Andrea Arcangeli <aarcange@redhat.com>, Michal Hocko <mhocko@suse.com>, Johannes Weiner <hannes@cmpxchg.org>, Shaohua Li <shli@kernel.org>, Hugh Dickins <hughd@google.com>, Minchan Kim <minchan@kernel.org>, Rik van Riel <riel@redhat.com>, Dave Hansen <dave.hansen@linux.intel.com>, Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>, Zi Yan <zi.yan@cs.rutgers.edu>

Daniel Jordan <daniel.m.jordan@oracle.com> 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;