linux-kernel - Re: [PATCH 7/8] zswap: add to mm/

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Date:	Fri, 04 Jan 2013 09:42:26 -0600
From:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nitin Gupta <ngupta@...are.org>,
	Minchan Kim <minchan@...nel.org>,
	Konrad Wilk <konrad.wilk@...cle.com>,
	Robert Jennings <rcj@...ux.vnet.ibm.com>,
	Jenifer Hopper <jhopper@...ibm.com>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <jweiner@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org,
	Dave Hansen <dave@...ux.vnet.ibm.com>
Subject: Re: [PATCH 7/8] zswap: add to mm/

On 01/03/2013 04:33 PM, Dan Magenheimer wrote:
>> From: Seth Jennings [mailto:sjenning@...ux.vnet.ibm.com]
>>
>> However, once the flushing code was introduced and could free an entry
>> from the zswap_fs_store() path, it became necessary to add a per-entry
>> refcount to make sure that the entry isn't freed while another code
>> path was operating on it.
> 
> Hmmm... doesn't the refcount at least need to be an atomic_t?

An entry's refcount is only ever changed under the tree lock, so
making them atomic_t would be redundantly atomic.

I should add a comment to that effect though, including all elements
that are protected by the tree lock which include:
* the tree structure
* the lru list
* the per-entry refcounts

I'll put that change in the queue for v2.

> Also, how can you "free" any entry of an rbtree while another
> thread is walking the rbtree?  (Deleting an entry from an rbtree
> causes rebalancing... afaik there is no equivalent RCU
> implementation for rbtrees... not that RCU would necessarily
> work well for this anyway.)

This also can't happen since a thread must obtain the tree lock before
accessing or changing the tree.

Regarding RCU, I saw that some work had been done on RCU aware rbtree
functions but they weren't ready yet.

> BTW, in case it appears otherwise, I'm trying to be helpful, not
> critical.  In the end, I think we are in agreement that in-kernel
> compression is very important and that the frontswap (and/or
> cleancache) interface(s) are the right way to identify compressible
> data, and we are mostly arguing allocation and implementation details.

Yes. I'm always grateful for comments about the code :)  At the very
least, it rehashes the justifications for design decisions.

Thanks,
Seth

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