[1/6] swap: Add comments to lock_cluster_or_swap_info()

Commit Message

Huang, Ying July 12, 2018, 11:36 p.m. UTC

From: Huang Ying <[e-mail]>

To improve the code readability.

Signed-off-by: "Huang, Ying" <[e-mail]>
Suggested-by: Dave Hansen <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Shaohua Li <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Minchan Kim <[e-mail]>
Cc: Rik van Riel <[e-mail]>
Cc: Daniel Jordan <[e-mail]>
Cc: Dan Williams <[e-mail]>
---
 mm/swapfile.c | 6 ++++++
 1 file changed, 6 insertions(+)

Comments

Dave Hansen July 13, 2018, 10:48 a.m. UTC | #1

> +/*
> + * 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.

Huang, Ying July 14, 2018, 4:07 a.m. UTC | #2

Dave Hansen <[e-mail]> 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

Patch

diff --git a/mm/swapfile.c b/mm/swapfile.c
index d8fddfb000ec..e31aa601d9c0 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -297,6 +297,12 @@  static inline void unlock_cluster(struct swap_cluster_info *ci)
 		spin_unlock(&ci->lock);
 }
 
+/*
+ * At most times, fine grained cluster lock is sufficient to protect
+ * the operations on sis->swap_map.  No need to acquire gross grained
+ * 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)