Re: [PATCH v2 10/10] zram: use atomic64_xxx() to replace zram_stat64_xxx()

[Jerome Marchand <jmarchan@redhat.com>]

On 06/05/2013 06:21 PM, Jiang Liu wrote:
> On Wed 05 Jun 2013 08:02:12 PM CST, Jerome Marchand wrote:
>> On 06/04/2013 06:06 PM, Jiang Liu wrote:
>>> Use atomic64_xxx() to replace open-coded zram_stat64_xxx().
>>> Some architectures have native support of atomic64 operations,
>>> so we can get rid of the spin_lock() in zram_stat64_xxx().
>>> On the other hand, for platforms use generic version of atomic64
>>> implement, it may cause an extra save/restore of the interrupt
>>> flag.  So it's a tradeoff.
>>>
>>> Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
>>
>> Before optimizing stats, I'd like to make sure that they're correct.
>> What makes 64 bits fields so different that they need atomicity while
>> 32 bits wouldn't? Actually all of them save compr_size only increase,
>> which would make a race less critical than for 32 bits fields that all
>> can go up and down (if a decrement overwrites a increment, the counter
>> can wrap around zero).
>>
>> Jerome
>>
> Hi Jerome,
>           I'm not sure about the design decision, but I could give a 
> guess here.
> 1) All 32-bit counters are only modified by 
> zram_bvec_write()/zram_page_free()
> and is/should be protected by down_write(&zram->lock).

Good point!

> 2) __zram_make_request() modifies some 64-bit counters without 
> protection.
> 3) zram_bvec_write() modifies some 64-bit counters and it's protected 
> with
>      down_read(&zram->lock).

I assume you mean down_write().

> 4) It's always safe for sysfs handler to read 32bit counters.
> 5) It's unsafe for sysfs handler to read 64bit counters on 32bit 
> platforms.

I was unaware of that.

> 
> So it does work with current design, but very hard to understand.
> Suggest to use atomic_t for 32bit counters too for maintainability,
> though may be a little slower.
> Any suggestion?

If atomic counter aren't necessary, no need to use them, but a comment
in zram_stats definition would be nice. Could you add one in your next
version of this patch?

Thanks
Jerome

> Regards!
> Gerry
>