linux-kernel - RE: [GIT PULL] mm: frontswap (for 3.2 window)

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Date:	Sun, 30 Oct 2011 13:06:02 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
Cc:	John Stoffel <john@...ffel.org>,
	Johannes Weiner <jweiner@...hat.com>,
	Pekka Enberg <penberg@...nel.org>,
	Cyclonus J <cyclonusj@...il.com>,
	Sasha Levin <levinsasha928@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	David Rientjes <rientjes@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Konrad Wilk <konrad.wilk@...cle.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>, ngupta@...are.org,
	Chris Mason <chris.mason@...cle.com>, JBeulich@...ell.com,
	Jonathan Corbet <corbet@....net>
Subject: RE: [GIT PULL] mm: frontswap (for 3.2 window)

On Sun, 2011-10-30 at 12:18 -0700, Dan Magenheimer wrote:
> > since they're the ones who will have to understand this stuff and know
> > how to maintain it.  And keeping this maintainable is a key goal.
> 
> Absolutely agree.  Count the number of frontswap lines that affect
> the current VM core code and note also how they are very clearly
> identified.  It really is a very VERY small impact to the core VM
> code (e.g. in the files swapfile.c and page_io.c). 

Granted, the impact on the core VM in lines of code is small.  But, I
think the behavioral impact is potentially huge since tmem's hooks add
non-trivial amounts of framework underneath the VM in core paths.  In
zcache's case, this means a bunch of allocations and an entirely new
allocator memory allocator being used in the swap paths.

We're certainly still shaking bugs out of the interactions there like
with zcache_direct_reclaim_lock.  Granted, that's not a
tmem/frontswap/cleancache bug, but it does speak to the difficulty and
subtlety of writing one of those frameworks underneath the tmem API.

-- Dave

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