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

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Date:   Fri, 13 Jul 2018 03:48:28 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     "Huang, Ying" <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     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()

> +/*
> + * At most times, fine grained cluster lock is sufficient to protect

Can we call out those times, please?

> + * 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".

> + * 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.