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From: Srivathsa Dara <srivathsa.d.dara@oracle.com>
To: Maximilian Heyne <mheyne@amazon.de>
CC: "ravib@amazon.com" <ravib@amazon.com>, Theodore Ts'o <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        "linux-ext4@vger.kernel.org"
	<linux-ext4@vger.kernel.org>,
        "linux-kernel@vger.kernel.org"
	<linux-kernel@vger.kernel.org>,
        Rajesh Sivaramasubramaniom
	<rajesh.sivaramasubramaniom@oracle.com>,
        Junxiao Bi <junxiao.bi@oracle.com>
Subject: RE: [External] : [PATCH] ext4: fix corruption during on-line resize
Thread-Topic: [External] : [PATCH] ext4: fix corruption during on-line resize
Thread-Index: AQHaWPVja2Juw9htvkCjWHcAtaz45bELXOww
Date: Thu, 15 Feb 2024 12:34:01 +0000
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References: <20240206120504.122870-1-mheyne@amazon.de>
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On Tue, 2024-02-06 at 12:05 +0000, Maximilian Heyne wrote:
> We observed a corruption during on-line resize of a file system that is
> larger than 16 TiB with 4k block size. With having more then 2^32 blocks
> resize_inode is turned off by default by mke2fs. The issue can be
> reproduced on a smaller file system for convenience by explicitly
> turning off resize_inode. An on-line resize across an 8 GiB boundary (the
> size of a meta block group in this setup) then leads to a corruption:
>
>   dev=3D/dev/<some_dev> # should be >=3D 16 GiB
>   mkdir -p /corruption
>   /sbin/mke2fs -t ext4 -b 4096 -O ^resize_inode $dev $((2 * 2**21 - 2**15=
))
>   mount -t ext4 $dev /corruption
>
>   dd if=3D/dev/zero bs=3D4096 of=3D/corruption/test count=3D$((2*2**21 - =
4*2**15))
Tip: Use fallocate for faster allocations.

>   sha1sum /corruption/test
>   # 79d2658b39dcfd77274e435b0934028adafaab11  /corruption/test
>
>   /sbin/resize2fs $dev $((2*2**21))
>   # drop page cache to force reload the block from disk
>   echo 1 > /proc/sys/vm/drop_caches
>
>   sha1sum /corruption/test
>   # 3c2abc63cbf1a94c9e6977e0fbd72cd832c4d5c3  /corruption/test
>
> 2^21 =3D 2^15*2^6 equals 8 GiB whereof 2^15 is the number of blocks per
> block group and 2^6 are the number of block groups that make a meta
> block group.
>
> The last checksum might be different depending on how the file is laid
> out across the physical blocks. The actual corruption occurs at physical
> block 63*2^15 =3D 2064384 which would be the location of the backup of th=
e
> meta block group's block descriptor. During the on-line resize the file
> system will be converted to meta_bg starting at s_first_meta_bg which is
> 2 in the example - meaning all block groups after 16 GiB. However, in
> ext4_flex_group_add we might add block groups that are not part of the
> first meta block group yet. In the reproducer we achieved this by
> substracting the size of a whole block group from the point where the
> meta block group would start. This must be considered when updating the
> backup block group descriptors to follow the non-meta_bg layout. The fix
> is to add a test whether the group to add is already part of the meta
> block group or not.
>
> Signed-off-by: Maximilian Heyne <mheyne@amazon.de>
> ---
>  fs/ext4/resize.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
> index 4d4a5a32e310..3433e83bb7a3 100644
> --- a/fs/ext4/resize.c
> +++ b/fs/ext4/resize.c
> @@ -1602,7 +1602,9 @@ static int ext4_flex_group_add(struct super_block *=
sb,
>                 int gdb_num =3D group / EXT4_DESC_PER_BLOCK(sb);
>                 int gdb_num_end =3D ((group + flex_gd->count - 1) /
>                                    EXT4_DESC_PER_BLOCK(sb));
> -               int meta_bg =3D ext4_has_feature_meta_bg(sb);
> +               unsigned long metagroup =3D group / EXT4_DESC_PER_BLOCK(s=
b);
> +               int meta_bg =3D ext4_has_feature_meta_bg(sb) &&
> +                             metagroup >=3D le32_to_cpu(es->s_first_meta=
_bg);

Instead of creating a new variable, please use existing variable `gdb_num`.

>                 sector_t padding_blocks =3D meta_bg ? 0 : sbi->s_sbh->b_b=
locknr -
>                                          ext4_group_first_block_no(sb, 0)=
;
> --
> 2.40.1

Have tested it on different filesystem setups, it does stop corruption.

>
>
>
>
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