linux-ext4 - Re: Regular ext4 error warning with HD in USB dock

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Date:	Fri, 7 Jan 2011 23:07:23 +0200
From:	Amir Goldstein <amir73il@...il.com>
To:	"Ted Ts'o" <tytso@....edu>
Cc:	Ric Wheeler <rwheeler@...hat.com>,
	Rogier Wolff <R.E.Wolff@...wizard.nl>,
	Con Kolivas <kernel@...ivas.org>, adilger.kernel@...ger.ca,
	linux-ext4@...r.kernel.org
Subject: Re: Regular ext4 error warning with HD in USB dock

On Fri, Jan 7, 2011 at 9:41 PM, Amir Goldstein <amir73il@...il.com> wrote:
> On Fri, Jan 7, 2011 at 7:26 AM, Ted Ts'o <tytso@....edu> wrote:
>> Am I missing something?  The kernel stores up to 3k worth of data, on
>> a 4k block file system.  Whereas e2fsck patch blindly assume 2k worhth
>> of data regardless of the block size.  The kernel patch looks ok, but
>> the e2fsprogs patch seems badly broken....

So it's not badly broken, it copies blocksize-2K, which is clumsily
written like this:
+	int len = ctx->fs->blocksize - 2*SUPERBLOCK_OFFSET;

After that, only 4K block and 8K block will have a leftover,
which will be copied from journal sb+1K to ext4 sb+1K.
Yes, 2K blocks - no message buffer for you!

The reason I am only copying 2K and throwing the extra K is this:
print_message_buffer() is called from check_super_block(),
*after* journal recovery, which was executed either by e2fsck itself
or (and more likely in a errors=remount-ro situation) by ext4
on read-only mount.
In the latter case, the extra K must be discarded, so I saw no reason
to write special code for the first case.
Neither did I find a good reason to complicate the recording code
and limit it to record only blocksize-2K.

>>
>>                                                - Ted
>>
>
> I thought that ext4 super block on 4K block is 1K at offset 1K, so
> kernel only uses the remaining 2K.
>
> It may very well be that e2fsck patch only works with 4K block size properly,
> because I never tested it on other block sizes.
>
> Amir.
>
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