Re: [PATCH 2/4] zcache: zero-filled pages awareness

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

On Wed, Mar 13, 2013 at 09:53:48AM -0700, Dan Magenheimer wrote:
>> From: Wanpeng Li [mailto:liwanp@linux.vnet.ibm.com]
>> Subject: [PATCH 2/4] zcache: zero-filled pages awareness
>> 
>> Compression of zero-filled pages can unneccessarily cause internal
>> fragmentation, and thus waste memory. This special case can be
>> optimized.
>> 
>> This patch captures zero-filled pages, and marks their corresponding
>> zcache backing page entry as zero-filled. Whenever such zero-filled
>> page is retrieved, we fill the page frame with zero.
>> 
>> Signed-off-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
>> ---
>>  drivers/staging/zcache/tmem.c        |    4 +-
>>  drivers/staging/zcache/tmem.h        |    5 ++
>>  drivers/staging/zcache/zcache-main.c |   87 ++++++++++++++++++++++++++++++----
>>  3 files changed, 85 insertions(+), 11 deletions(-)
>> 
>> diff --git a/drivers/staging/zcache/tmem.c b/drivers/staging/zcache/tmem.c
>> index a2b7e03..62468ea 100644
>> --- a/drivers/staging/zcache/tmem.c
>> +++ b/drivers/staging/zcache/tmem.c
>> @@ -597,7 +597,9 @@ int tmem_put(struct tmem_pool *pool, struct tmem_oid *oidp, uint32_t index,
>>  	if (unlikely(ret == -ENOMEM))
>>  		/* may have partially built objnode tree ("stump") */
>>  		goto delete_and_free;
>> -	(*tmem_pamops.create_finish)(pampd, is_ephemeral(pool));
>> +	if (pampd != (void *)ZERO_FILLED)
>> +		(*tmem_pamops.create_finish)(pampd, is_ephemeral(pool));
>> +
>>  	goto out;
>> 
>>  delete_and_free:
>> diff --git a/drivers/staging/zcache/tmem.h b/drivers/staging/zcache/tmem.h
>> index adbe5a8..6719dbd 100644
>> --- a/drivers/staging/zcache/tmem.h
>> +++ b/drivers/staging/zcache/tmem.h
>> @@ -204,6 +204,11 @@ struct tmem_handle {
>>  	uint16_t client_id;
>>  };
>> 
>> +/*
>> + * mark pampd to special vaule in order that later
>> + * retrieve will identify zero-filled pages
>> + */
>> +#define ZERO_FILLED 0x2
>
>You can avoid changing tmem.[ch] entirely by moving this
>definition into zcache-main.c and by moving the check
>comparing pampd against ZERO_FILLED into zcache_pampd_create_finish()
>I think that would be cleaner...

Great point!

>
>If you change this and make the pageframe counter fix for PATCH 4/4,
>please add my ack for the next version:
>

Thanks Dan. :-)

Regards,
Wanpeng Li 

>Acked-by: Dan Magenheimer <dan.magenheimer@oracle.com>
>
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