From: Lukas Czerner <lczerner@redhat.com>
Subject: Re: [PATCH] ext4: Fix max file size of extent format file
Date: Fri, 13 May 2011 08:34:42 +0200 (CEST)
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Cc: ext4 <linux-ext4@vger.kernel.org>, Theodore Tso <tytso@mit.edu>
To: Kazuya Mio <k-mio@sx.jp.nec.com>
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On Wed, 11 May 2011, Kazuya Mio wrote:

> We hit BUG_ON in ext4_ext_put_gap_in_cache() while creating a file
> whose size is the max file size of extent format. Because the extent cache
> length is 0 when we allocate two blocks to the file offset 2^32-2, and then
> the offset 2^32-1. To fix it, we decrease the max file size to
> (2^32-2)*blocksize. In this way, we would be able to allocate a block up to
> the offset 2^32-2. I think there is no data loss because we can read all files
> created before applying this patch.
> 
> How to reproduce:
> I'm running 2.6.39-rc6. Note that i386 architecture and 4KB blocksize cannot
> reproduce this problem.
> 
> # dd if=/dev/zero of=/mnt/mp1/file bs=<blocksize> count=1 seek=$((2**32-2))
> # sync
> # dd if=/dev/zero of=/mnt/mp1/file bs=<blocksize> count=1 seek=$((2**32-1))

Hi Kazuya,

Thanks for the patch, however I think that there is a better solution
than lowering the max file size, which is not necessary. I would rather
fix ext4_ext_put_gap_in_cache() and allow to invalidate the cache by
setting the length to zero.

Please see the following patch, if it works for you.

Thanks!
-Lukas

> 
> Signed-off-by: Kazuya Mio <k-mio@sx.jp.nec.com>
> ---
>  fs/ext4/super.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 8553dfb..fce0249 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -2248,8 +2248,8 @@ static loff_t ext4_max_size(int blkbits, int has_huge_files)
>  
>  	/* 32-bit extent-start container, ee_block */
>  	res = 1LL << 32;
> -	res <<= blkbits;
>  	res -= 1;
> +	res <<= blkbits;
>  
>  	/* Sanity check against vm- & vfs- imposed limits */
>  	if (res > upper_limit)
> 
> --
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