linux-kernel - Re: [PATCH] [staging][zram] Fix handling of incompressible pages

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Date:	Mon, 22 Oct 2012 13:43:08 -0700
From:	Greg KH <greg@...ah.com>
To:	Nitin Gupta <ngupta@...are.org>
Cc:	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Dan Carpenter <dan.carpenter@...cle.com>,
	Sam Hansen <solid.se7en@...il.com>,
	Linux Driver Project <devel@...uxdriverproject.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [staging][zram] Fix handling of incompressible pages

On Wed, Oct 10, 2012 at 05:42:18PM -0700, Nitin Gupta wrote:
> Change 130f315a (staging: zram: remove special handle of uncompressed page)
> introduced a bug in the handling of incompressible pages which resulted in
> memory allocation failure for such pages.
> 
> When a page expands on compression, say from 4K to 4K+30, we were trying to
> do zsmalloc(pool, 4K+30). However, the maximum size which zsmalloc can
> allocate is PAGE_SIZE (for obvious reasons), so such allocation requests
> always return failure (0).
> 
> For a page that has compressed size larger than the original size (this may
> happen with already compressed or random data), there is no point storing
> the compressed version as that would take more space and would also require
> time for decompression when needed again. So, the fix is to store any page,
> whose compressed size exceeds a threshold (max_zpage_size), as-it-is i.e.
> without compression.  Memory required for storing this uncompressed page can
> then be requested from zsmalloc which supports PAGE_SIZE sized allocations.
> 
> Lastly, the fix checks that we do not attempt to "decompress" the page which
> we stored in the uncompressed form -- we just memcpy() out such pages.

So this fix needs to go to the stable 3.6 release also, right?

thanks,

greg k-h
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