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

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Date:   Fri, 06 Jul 2018 20:49:05 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Daniel Jordan <daniel.m.jordan@...cle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        "Shaohua Li" <shli@...nel.org>, Hugh Dickins <hughd@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Zi Yan <zi.yan@...rutgers.edu>
Subject: Re: [PATCH -mm -v4 05/21] mm, THP, swap: Support PMD swap mapping in free_swap_and_cache()/swap_free()

Daniel Jordan <daniel.m.jordan@...cle.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;