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

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Message-ID: <CAPkvG_eVF-ZiwLr=i9gRodqfrSuehSoMOAL9hkpUBtOFRXE8LQ@mail.gmail.com>
Date:	Mon, 22 Oct 2012 14:48:03 -0700
From:	Nitin Gupta <ngupta@...are.org>
To:	Greg KH <greg@...ah.com>
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 Mon, Oct 22, 2012 at 1:43 PM, Greg KH <greg@...ah.com> wrote:
> 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?
>

Forgot to mention -- yes, this needs to be in 3.6 also.

Thanks,
Nitin
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