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Date:   Tue, 7 Feb 2017 10:20:57 +0800
From:   zhouxianrong <zhouxianrong@...wei.com>
To:     Minchan Kim <minchan@...nel.org>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <akpm@...ux-foundation.org>, <sergey.senozhatsky@...il.com>,
        <willy@...radead.org>, <iamjoonsoo.kim@....com>,
        <ngupta@...are.org>, <Mi.Sophia.Wang@...wei.com>,
        <zhouxiyu@...wei.com>, <weidu.du@...wei.com>,
        <zhangshiming5@...wei.com>, <won.ho.park@...wei.com>
Subject: Re: [PATCH] mm: extend zero pages to same element pages for zram



On 2017/2/7 7:48, Minchan Kim wrote:
> Hi
>
> On Mon, Feb 06, 2017 at 09:28:18AM +0800, zhouxianrong wrote:
>>
>>
>> On 2017/2/5 22:21, Minchan Kim wrote:
>>> Hi zhouxianrong,
>>>
>>> On Fri, Feb 03, 2017 at 04:42:27PM +0800, zhouxianrong@...wei.com wrote:
>>>> From: zhouxianrong <zhouxianrong@...wei.com>
>>>>
>>>> test result as listed below:
>>>>
>>>> zero   pattern_char pattern_short pattern_int pattern_long total   (unit)
>>>> 162989 14454        3534          23516       2769         3294399 (page)
>>>>
>>>> statistics for the result:
>>>>
>>>>        zero  pattern_char  pattern_short  pattern_int  pattern_long
>>>> AVERAGE 0.745696298 0.085937175 0.015957701 0.131874915 0.020533911
>>>> STDEV   0.035623777 0.016892402 0.004454534 0.021657123 0.019420072
>>>> MAX     0.973813421 0.222222222 0.021409518 0.211812245 0.176512625
>>>> MIN     0.645431905 0.004634398 0           0           0
>>>
>>> The description in old version was better for justifying same page merging
>>> feature.
>>>
>>>>
>>>> Signed-off-by: zhouxianrong <zhouxianrong@...wei.com>
>>>> ---
>>>> drivers/block/zram/zram_drv.c |  124 +++++++++++++++++++++++++++++++----------
>>>> drivers/block/zram/zram_drv.h |   11 ++--
>>>> 2 files changed, 103 insertions(+), 32 deletions(-)
>>>>
>>>> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
>>>> index e5ab7d9..6a8c9c5 100644
>>>> --- a/drivers/block/zram/zram_drv.c
>>>> +++ b/drivers/block/zram/zram_drv.c
>>>> @@ -95,6 +95,17 @@ static void zram_clear_flag(struct zram_meta *meta, u32 index,
>>>> 	meta->table[index].value &= ~BIT(flag);
>>>> }
>>>>
>>>> +static inline void zram_set_element(struct zram_meta *meta, u32 index,
>>>> +			unsigned long element)
>>>> +{
>>>> +	meta->table[index].element = element;
>>>> +}
>>>> +
>>>> +static inline void zram_clear_element(struct zram_meta *meta, u32 index)
>>>> +{
>>>> +	meta->table[index].element = 0;
>>>> +}
>>>> +
>>>> static size_t zram_get_obj_size(struct zram_meta *meta, u32 index)
>>>> {
>>>> 	return meta->table[index].value & (BIT(ZRAM_FLAG_SHIFT) - 1);
>>>> @@ -167,31 +178,78 @@ static inline void update_used_max(struct zram *zram,
>>>> 	} while (old_max != cur_max);
>>>> }
>>>>
>>>> -static bool page_zero_filled(void *ptr)
>>>> +static inline void zram_fill_page(char *ptr, unsigned long value)
>>>> +{
>>>> +	int i;
>>>> +	unsigned long *page = (unsigned long *)ptr;
>>>> +
>>>> +	if (likely(value == 0)) {
>>>> +		clear_page(ptr);
>>>> +	} else {
>>>> +		for (i = 0; i < PAGE_SIZE / sizeof(*page); i++)
>>>> +			page[i] = value;
>>>> +	}
>>>> +}
>>>> +
>>>> +static inline void zram_fill_page_partial(char *ptr, unsigned int size,
>>>> +		unsigned long value)
>>>> +{
>>>> +	int i;
>>>> +	unsigned long *page;
>>>> +
>>>> +	if (likely(value == 0)) {
>>>> +		memset(ptr, 0, size);
>>>> +		return;
>>>> +	}
>>>> +
>>>> +	i = ((unsigned long)ptr) % sizeof(*page);
>>>> +	if (i) {
>>>> +		while (i < sizeof(*page)) {
>>>> +			*ptr++ = (value >> (i * 8)) & 0xff;
>>>> +			--size;
>>>> +			++i;
>>>> +		}
>>>> +	}
>>>> +
>>>
>>> I don't think we need this part because block layer works with sector
>>> size or multiple times of it so it must be aligned unsigned long.
>>>
>>>
>>>
>>>
>>> .
>>>
>>
>> Minchan and Matthew Wilcox:
>>
>> 1. right, but users could open /dev/block/zram0 file and do any read operations.
>
> Could you make that happen?
> I don't think it's possible as Matthew already pointed out, too.

yes, Matthew is right, thanks

>
>>
>> 2. about endian operation for long, the modification is trivial and low efficient.
>>    i have not better method. do you have any good idea for this?
>
> So if assumption 1 is wrong, we don't need 2, either.

yes

>
>>
>> 3. the below should be modified.
>>
>> static inline bool zram_meta_get(struct zram *zram)
>> @@ -495,11 +553,17 @@ static void zram_meta_free(struct zram_meta *meta, u64 disksize)
>>
>>  	/* Free all pages that are still in this zram device */
>>  	for (index = 0; index < num_pages; index++) {
>> -		unsigned long handle = meta->table[index].handle;
>> +		unsigned long handle;
>> +
>> +		bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
>> +		handle = meta->table[index].handle;
>>
>> -		if (!handle)
>> +		if (!handle || zram_test_flag(meta, index, ZRAM_SAME)) {
>> +			bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
>>  			continue;
>> +		}
>>
>> +		bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
>>  		zs_free(meta->mem_pool, handle);
>
> Could you explain why we need this modification?
>
>>  	}
>>
>> @@ -511,7 +575,7 @@ static void zram_meta_free(struct zram_meta *meta, u64 disksize)
>>  static struct zram_meta *zram_meta_alloc(char *pool_name, u64 disksize)
>>  {
>>  	size_t num_pages;
>> -	struct zram_meta *meta = kmalloc(sizeof(*meta), GFP_KERNEL);
>> +	struct zram_meta *meta = kzalloc(sizeof(*meta), GFP_KERNEL);
>
> Ditto
>
>>
>>
>
> .
>

because of union of handle and element, i think a non-zero element (other than handle) is prevented from freeing.
if zram_meta_get was modified, zram_meta_alloc did so.

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