linux-kernel - Re: lz4hc compression in UBIFS?

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Date:	Wed, 23 Oct 2013 00:26:16 -0500
From:	Brent Taylor <motobud@...il.com>
To:	Konstantin Tokarev <annulen@...dex.ru>
Cc:	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
	Artem Bityutskiy <dedekind1@...il.com>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: lz4hc compression in UBIFS?

Konstantin,
   I did my testing with data from /dev/urandom (which I now realize
wasn't the best choice of data source), but if I use /dev/zero (which
actually causes data compression to occur), the decompressor fails.  I
don't know the internal workings of the lz4hc compressor or the lz4
decompressor.  I couldn't find any examples of any code in the kernel
actually using the compressor.  I've cc'ed the maintainers of the
lz4hc_compress.c to see if they my have some more insight to the
issue.

-- Brent

On Tue, Oct 22, 2013 at 5:10 AM, Konstantin Tokarev <annulen@...dex.ru> wrote:
>
>
> 22.10.2013, 07:43, "Brent Taylor" <motobud@...il.com>:
>> On Mon, Oct 21, 2013 at 10:59 AM, Konstantin Tokarev <annulen@...dex.ru> wrote:
>>
>>>  04.10.2013, 07:09, "Brent Taylor" <motobud@...il.com>:
>>>>  Here is a patch based on linux-3.12-rc3.  I haven't performed any
>>>>  performance testing UBIFS using lz4hc, but I can mount UBIFS volumes
>>>>  and haven't seen any problems yet.  The only think I know that isn't
>>>>  correct about the patch is the description for the Kconfig element for
>>>>  select lz4hc as a compression option.  I only copied the description
>>>>  from the lzo description.
>>>  Hi Brent,
>>>
>>>  I'm testing your patch on my SH4 device. When I create new partition
>>>  with lz4hc compressor, it works fine: I can copy file into it, and
>>>  md5sums of original and copy match. However, after reboot I cannot
>>>  read the file anymore:
>>>
>>>  UBIFS error (pid 1101): ubifs_decompress: cannot decompress 934 bytes, compressor lz4hc, error -22
>>>  UBIFS error (pid 1101): read_block: bad data node (block 1, inode 65)
>>>  UBIFS error (pid 1101): do_readpage: cannot read page 1 of inode 65, error -22
>>>
>>>  The same error appears if I use lz4hc-compressed ubifs image to flash rootfs
>>>  (using patched mkfs.ubifs).
>>>
>>>  Decompression error occurs in lz4_uncompress() function (lib/lz4/lz4_decompress.c),
>>>  on the line 101:
>>>
>>>  /* Error: offset create reference outside destination buffer */
>>>  if (unlikely(ref < (BYTE *const) dest))
>>>      goto _output_error;
>>>
>>>  Brent: are you able to read data from lz4hc volume on your device?
>>>  Anyone: any ideas what may happen here?
>>>
>>>  --
>>>  Regards,
>>>  Konstantin
>>
>> Konstantin,
>>    I haven't seen anything like that on my at91sam9m10g45-ek
>> development board.  I haven't used a flash image from mkfs.ubifs yet.
>> Is it possible the file system was not umounted cleanly before the
>> reboot and UBIFS went through a recovery procedure?  Maybe something
>> breaks with lz4hc when UBIFS does a recovery?  That's just a guess.
>
> Could you save attached file on lz4hc volume, umount it and mount again?
> I get aforementioned error when doing `cat set11.cfg`
>
> --
> Regards,
> Konstantin
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