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

[Jiang Liu <liuj97@gmail.com>]

On Thu 06 Jun 2013 05:37:19 PM CST, Jerome Marchand wrote:
> 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().
Actually I mean "zram_bvec_read()" instead of "zram_bvec_write()".
Read side is protected by down_read(&zram->lock).
Regards!
Gerry

>
>> 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?
Sure!

>
> Thanks
> Jerome
>
>> Regards!
>> Gerry
>>
>