linux-ext4 - Re: [PATCH] fix bogus BUG

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Message-ID: <20090313003855.GM17104@mit.edu>
Date:	Thu, 12 Mar 2009 20:38:55 -0400
From:	Theodore Tso <tytso@....edu>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Thiemo Nagel <thiemo.nagel@...tum.de>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] fix bogus BUG_ONs in in mballoc code

On Thu, Mar 12, 2009 at 01:46:57PM -0500, Eric Sandeen wrote:
> Thiemo Nagel reported that:
> 
> # dd if=/dev/zero of=image.ext4 bs=1M count=2
> # mkfs.ext4 -v -F -b 1024 -m 0 -g 512 -G 4 -I 128 -N 1 \
>   -O large_file,dir_index,flex_bg,extent,sparse_super image.ext4
> # mount -o loop image.ext4 mnt/
> # dd if=/dev/zero of=mnt/file
> 
> oopsed, with a BUG_ON in ext4_mb_normalize_request because
> size == EXT4_BLOCKS_PER_GROUP
> 
> It appears to me (esp. after talking to Andreas) that the BUG_ON
> is bogus; a request of exactly EXT4_BLOCKS_PER_GROUP should
> be allowed, though larger sizes do indicate a problem.

Clearly we should make this change to avoid the BUG_ON; but stupid
question, why shouldn't we allow sizes larger than
EXT4_BLOCKS_PER_GROUP?  

Especially with flex_bg, it is possible for an allocation size >
EXT4_BLOCKS_PER_GROUP to be satisifed, especially if the filesystem
isn't that full yet, and it might even make sense to request a larger
allocation for video files that are getting preallocated, for
example....

						- Ted
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