linux-kernel - Re: [PATCH 1/6] swap: Add comments to lock_cluster_or_swap

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Date:   Sat, 14 Jul 2018 12:07:43 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, 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>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 1/6] swap: Add comments to lock_cluster_or_swap_info()

Dave Hansen <dave.hansen@...ux.intel.com> writes:

>> +/*
>> + * At most times, fine grained cluster lock is sufficient to protect
>
> Can we call out those times, please?

To protect si->swap_map[], if HDD, si->lock is used, otherwise cluster
lock is used.  "at most times" is ambiguous here, I will fix it.

>> + * the operations on sis->swap_map.  
>
> Please be careful with the naming.  You can call it 'si' because that's
> what the function argument is named.  Or, swap_info_struct because
> that's the struct name.  Calling it 'sis' is a bit sloppy, no?
>
>> 					No need to acquire gross grained
>
> "coarse" is a conventional antonym for "fine".

Sorry for my poor English, will change this.

>> + * sis->lock.  But cluster and cluster lock isn't available for HDD,
>> + * so sis->lock will be instead for them.
>> + */
>>  static inline struct swap_cluster_info *lock_cluster_or_swap_info(
>>  	struct swap_info_struct *si,
>>  	unsigned long offset)
>
> What I already knew was: there are two locks.  We use one sometimes and
> the other at other times.
>
> What I don't know is why there are two locks, and the heuristics why we
> choose between them.  This comment doesn't help explain the things I
> don't know.

cluster lock is used to protect fields of struct swap_cluster_info, and
si->swap_map[], this is described in comments of struct
swap_cluster_info.  si->lock is used to protect other fields of si.  If
two locks need to be held, hold si->lock first.  This is for non-HDD.
For HDD, there are no cluster, so si->lock is used to protect
si->swap_map[].

Best Regards,
Huang, Ying