linux-kernel - Re: [PATCH 0/3] ramzswap: Eliminate stale data in compressed memory

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Date:	Thu, 11 Mar 2010 07:22:05 +0000 (GMT)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	Nitin Gupta <ngupta@...are.org>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg KH <greg@...ah.com>, Pekka Enberg <penberg@...il.com>,
	andi@...stfloor.org, linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] ramzswap: Eliminate stale data in compressed
 memory

On Fri, 5 Mar 2010, Nitin Gupta wrote:

> ramzswap driver creates RAM based block devices which can be
> used (only) as swap disks. Pages swapped to these disks are
> compressed and stored in memory itself.
> 
> However, these devices do not get any notification when a swap
> slot is freed (swap_map[i] reaches 0). So, we cannot free memory
> allocated corresponding to this swap slot. Such stale data can
> quickly accumulate in (compressed) memory defeating the whole
> purpose of such devices.
> 
> To overcome this problem, we now add a callback in 'struct swap_info_struct'
> which is called as soon as a swap slot is freed.
> 
> Adding handler for this callback:
> 	swapon notifier --> set_swap_free_notify(swap_type, fn)
> 
> Removing handler:
> 	swapoff notifier --> set_swap_free_notify(swap_type, NULL)
> 
> 
> Alternative approaches:
> 1) Add callback directly in 'struct block_device_operations' but
> that is considered too hacky.
> 2) Use swap discard mechanism: It involves unncessary overhead of
> allocating 'discard bio' requests and its too slow to serve ramzswap
> needs.
> 
>  drivers/staging/ramzswap/ramzswap_drv.c   |   91 +++++++++++++++++++++++++++++
>  drivers/staging/ramzswap/ramzswap_drv.h   |    1 +
>  drivers/staging/ramzswap/ramzswap_ioctl.h |    1 +
>  include/linux/swap.h                      |   16 +++++-
>  mm/swapfile.c                             |   78 ++++++++++++++++++++++++
>  5 files changed, 185 insertions(+), 2 deletions(-)

This is one of the various solutions I disliked already, isn't it?

To me this is just a more convoluted and obscure variant of the
block_device_operations swap_slot_free_notify patch you had in mmotm,
which Linus has rejected.

I admit, I did not understand at all what Linus was proposing with
readpage, writepage and address_space_operations: I kept quiet in
the hope that you'd understand where I didn't!

Hugh
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