Re: [PATCHv4 3/7] zswap: add to mm/

From: Seth Jennings <sjenning@linux.vnet.ibm.com>
To: Minchan Kim <minchan@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Nitin Gupta <ngupta@vflare.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Dan Magenheimer <dan.magenheimer@oracle.com>,
	Robert Jennings <rcj@linux.vnet.ibm.com>,
	Jenifer Hopper <jhopper@us.ibm.com>, Mel Gorman <mgorman@suse.de>,
	Johannes Weiner <jweiner@redhat.com>,
	Rik van Riel <riel@redhat.com>,
	Larry Woodman <lwoodman@redhat.com>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Dave Hansen <dave@linux.vnet.ibm.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	devel@driverdev.osuosl.org
Subject: Re: [PATCHv4 3/7] zswap: add to mm/
Date: Fri, 01 Feb 2013 09:31:09 -0600	[thread overview]
Message-ID: <510BDFBD.7090808@linux.vnet.ibm.com> (raw)
In-Reply-To: <20130201023821.GB6262@blaptop>

On 01/31/2013 08:38 PM, Minchan Kim wrote:
> On Thu, Jan 31, 2013 at 01:06:46PM -0600, Seth Jennings wrote:
>> On 01/31/2013 01:07 AM, Minchan Kim wrote:
>>> On Tue, Jan 29, 2013 at 03:40:23PM -0600, Seth Jennings wrote:
>>>> zswap is a thin compression backend for frontswap. It receives
>>>> pages from frontswap and attempts to store them in a compressed
>>>> memory pool, resulting in an effective partial memory reclaim and
>>>> dramatically reduced swap device I/O.
>>>>
>>>> Additionally, in most cases, pages can be retrieved from this
>>>> compressed store much more quickly than reading from tradition
>>>> swap devices resulting in faster performance for many workloads.
>>>>
>>>> This patch adds the zswap driver to mm/
>>>>
>>>> Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
>>>> ---
>>>>  mm/Kconfig  |  15 ++
>>>>  mm/Makefile |   1 +
>>>>  mm/zswap.c  | 656 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>>>>  3 files changed, 672 insertions(+)
>>>>  create mode 100644 mm/zswap.c
>>>>
>>>> diff --git a/mm/Kconfig b/mm/Kconfig
>>>> index 278e3ab..14b9acb 100644
>>>> --- a/mm/Kconfig
>>>> +++ b/mm/Kconfig
>>>> @@ -446,3 +446,18 @@ config FRONTSWAP
>>>>  	  and swap data is stored as normal on the matching swap device.
>>>>  
>>>>  	  If unsure, say Y to enable frontswap.
>>>> +
>>>> +config ZSWAP
>>>> +	bool "In-kernel swap page compression"
>>>> +	depends on FRONTSWAP && CRYPTO
>>>> +	select CRYPTO_LZO
>>>> +	select ZSMALLOC
>>>
>>> Again, I'm asking why zswap should have a dependent on CRPYTO?
>>> Couldn't we support it as a option? I'd like to use zswap without CRYPTO
>>> like zram.
>>
>> The reason we need CRYPTO is that zswap uses it to support a pluggable
>> compression model.  zswap can use any compressor that has a crypto API
>> driver.  zswap has _symbol dependencies_ on CRYPTO.  If it isn't
>> selected, the build breaks.
> 
> I think we can factor out compressoin part and remove dependency
> at compile time by Kconfig. No?

I'm still not following.  How would one "factor out" the crypto API
dependency when we use it to access the compressor modules.

The only thing I can think you're saying is to hack up the code with
ifdefs to call the lzo code directly based on a Kconfig option.  I
really hope you aren't saying that though :-/

> Of course, if we disable CRYPTO in Kconfig,
> we lost pluggable model but not a problem for embedded system.

The pluggable model is _very_ necessary for us because we use it to
access our hardware compression accelerator.  We do not use lzo in
that case.  We use 842 (crypto/842.c and drivers/crypto/nx/nx-842.c).

I'm not sure why we are misunderstanding on this.  Is there a specific
objection to depending the crypto API here? I understand that you are
thinking about embedded systems.  Does the enabling CRYPTO and
CRYPTO_LZO add significant size to the kernel or something?  Just
trying to understand why this is a problem.

Thanks,
Seth

> 
> Anyway, If it's a burden for you at a moment, I'm not going to insist on it.
> Will do it for myself.