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

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> 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...

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

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

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