Re: [PATCH v2 1/4] introduce zero filled pages handler

[Wanpeng Li <liwanp@linux.vnet.ibm.com>]

On Sat, Mar 16, 2013 at 09:03:04AM -0400, Konrad Rzeszutek Wilk wrote:
>On Thu, Mar 14, 2013 at 06:08:14PM +0800, Wanpeng Li wrote:
>> Introduce zero-filled pages handler to capture and handle zero pages.
>> 
>> Acked-by: Dan Magenheimer <dan.magenheimer@oracle.com>
>> Signed-off-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
>> ---
>>  drivers/staging/zcache/zcache-main.c |   26 ++++++++++++++++++++++++++
>>  1 files changed, 26 insertions(+), 0 deletions(-)
>> 
>> diff --git a/drivers/staging/zcache/zcache-main.c b/drivers/staging/zcache/zcache-main.c
>> index 328898e..b71e033 100644
>> --- a/drivers/staging/zcache/zcache-main.c
>> +++ b/drivers/staging/zcache/zcache-main.c
>> @@ -460,6 +460,32 @@ static void zcache_obj_free(struct tmem_obj *obj, struct tmem_pool *pool)
>>  	kmem_cache_free(zcache_obj_cache, obj);
>>  }
>>  
>> +static bool page_zero_filled(void *ptr)
>
>Shouldn't this be 'struct page *p' ?
>> +{
>> +	unsigned int pos;
>> +	unsigned long *page;
>> +
>> +	page = (unsigned long *)ptr;
>
>That way you can avoid this casting.

Great point! I will also clean it in zram implementation.

>> +
>> +	for (pos = 0; pos < PAGE_SIZE / sizeof(*page); pos++) {
>> +		if (page[pos])
>> +			return false;
>
>Perhaps allocate a static page filled with zeros and just do memcmp?
>> +	}
>> +
>> +	return true;
>> +}
>> +
>> +static void handle_zero_page(void *page)
>> +{
>> +	void *user_mem;
>> +
>> +	user_mem = kmap_atomic(page);
>> +	memset(user_mem, 0, PAGE_SIZE);
>> +	kunmap_atomic(user_mem);
>> +
>> +	flush_dcache_page(page);

To make sure kernel store is visiable to user space mappings of that page.

>
>This is new. Could you kindly explain why it is needed? Thanks.
>> +}
>> +
>>  static struct tmem_hostops zcache_hostops = {
>>  	.obj_alloc = zcache_obj_alloc,
>>  	.obj_free = zcache_obj_free,
>> -- 
>> 1.7.7.6
>> 

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