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Subject: RE: [GIT PULL] mm: frontswap (for 3.2 window)
From: Dave Hansen <dave@linux.vnet.ibm.com>
To: Dan Magenheimer <dan.magenheimer@oracle.com>
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Date: Sun, 30 Oct 2011 13:06:02 -0700
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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