On 2020/7/15 17:08, Johannes Thumshirn wrote:
> On 15/07/2020 11:03, Coly Li wrote:
>> On 2020/7/15 14:02, Hannes Reinecke wrote:
>>> On 7/15/20 7:45 AM, Coly Li wrote:
>>>> This patch adds comments to mark each member of struct cache_sb_disk,
>>>> it is helpful to understand the bcache superblock on-disk layout.
>>>>
>>>> Signed-off-by: Coly Li <colyli@xxxxxxx>
>>>> ---
>>>> include/uapi/linux/bcache.h | 39 +++++++++++++++++++------------------
>>>> 1 file changed, 20 insertions(+), 19 deletions(-)
>>>>
>>>> diff --git a/include/uapi/linux/bcache.h b/include/uapi/linux/bcache.h
>>>> index 9a1965c6c3d0..afbd1b56a661 100644
>>>> --- a/include/uapi/linux/bcache.h
>>>> +++ b/include/uapi/linux/bcache.h
>>>> @@ -158,33 +158,33 @@ static inline struct bkey *bkey_idx(const struct
>>>> bkey *k, unsigned int nr_keys)
>>>> #define BDEV_DATA_START_DEFAULT 16 /* sectors */
>>>> struct cache_sb_disk {
>>>> - __le64 csum;
>>>> - __le64 offset; /* sector where this sb was written */
>>>> - __le64 version;
>>>> +/*000*/ __le64 csum;
>>>> +/*008*/ __le64 offset; /* sector where this sb was
>>>> written */
>>>> +/*010*/ __le64 version;
>>>> - __u8 magic[16];
>>>> +/*018*/ __u8 magic[16];
>>>> - __u8 uuid[16];
>>>> +/*028*/ __u8 uuid[16];
>>>> union {
>>>> - __u8 set_uuid[16];
>>>> +/*038*/ __u8 set_uuid[16];
>>>> __le64 set_magic;
>>>> };
>>>> - __u8 label[SB_LABEL_SIZE];
>>>> +/*048*/ __u8 label[SB_LABEL_SIZE];
>>>> - __le64 flags;
>>>> - __le64 seq;
>>>> - __le64 pad[8];
>>>> +/*068*/ __le64 flags;
>>>> +/*070*/ __le64 seq;
>>>> +/*078*/ __le64 pad[8];
>>>> union {
>>>> struct {
>>>> /* Cache devices */
>>>> - __le64 nbuckets; /* device size */
>>>> +/*0b8*/ __le64 nbuckets; /* device size */
>>>> - __le16 block_size; /* sectors */
>>>> - __le16 bucket_size; /* sectors */
>>>> +/*0c0*/ __le16 block_size; /* sectors */
>>>> +/*0c2*/ __le16 bucket_size; /* sectors */
>>>> - __le16 nr_in_set;
>>>> - __le16 nr_this_dev;
>>>> +/*0c4*/ __le16 nr_in_set;
>>>> +/*0c6*/ __le16 nr_this_dev;
>>>> };
>>>> struct {
>>>> /* Backing devices */
>>>> @@ -198,14 +198,15 @@ struct cache_sb_disk {
>>>> };
>>>> };
>>>> - __le32 last_mount; /* time overflow in y2106 */
>>>> +/*0c8*/ __le32 last_mount; /* time overflow in y2106 */
>>>> - __le16 first_bucket;
>>>> +/*0cc*/ __le16 first_bucket;
>>>> union {
>>>> - __le16 njournal_buckets;
>>>> +/*0ce*/ __le16 njournal_buckets;
>>>> __le16 keys;
>>>> };
>>>> - __le64 d[SB_JOURNAL_BUCKETS]; /* journal buckets */
>>>> +/*0d0*/ __le64 d[SB_JOURNAL_BUCKETS]; /* journal
>>>> buckets */
>>>> +/*8d0*/
>>>> };
>>>> struct cache_sb {
>>>>
>>> Common practice is to place comments at the end; please don't use the
>>> start of the line here.
>>
>> Hi Hannes,
>>
>> When I try to move the offset comment to the line end, I find it
>> conflicts with normal code comment, e.g.
>> __le64 d[SB_JOURNAL_BUCKETS]; /* journal buckets */
>>
>> I have to add the offset comment to the line start. I guess this is why
>> ocfs2 code adds the offset comment at the line start.
>>
>> So finally I have to keep the offset comment on line start still...
>
> Why at them at all? pahole -C or crash/gdb will show them for you if you're
> interested and if you need it in the code you can use offsetof().
>
> I don't really see a good reason to add these comments.
>
You are right :-) With pahole there is no reason for having this patch.
Thanks for the informative hint, this patch will disappear in next
version series.
Coly Li
