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Date: Thu, 14 May 2020 18:21:20 -0400
From: Eric Whitney <enwlinux@...il.com>
To: Jeffle Xu <jefflexu@...ux.alibaba.com>
Cc: enwlinux@...il.com, linux-ext4@...r.kernel.org, tytso@....edu,
joseph.qi@...ux.alibaba.com
Subject: Re: [PATCH RFC] ext4: fix partial cluster initialization when
splitting extent
* Jeffle Xu <jefflexu@...ux.alibaba.com>:
> Hi Eric, would you mind explaining why the magic number '2' is used here
> when calculating the physical cluster number of the partial cluster in
> commit f4226d9ea400 ("ext4: fix partial cluster initialization") ?
>
> ```
> + /*
> + * If we're going to split the extent, note that
> + * the cluster containing the block after 'end' is
> + * in use to avoid freeing it when removing blocks.
> + */
> + if (sbi->s_cluster_ratio > 1) {
> + pblk = ext4_ext_pblock(ex) + end - ee_block + 2;
> + partial_cluster =
> + -(long long) EXT4_B2C(sbi, pblk);
> + }
> ```
>
> As far as I understand, there we are initializing the partial cluster
> describing the beginning of the split extent after 'end'. The
> corrsponding physical block number of the first block in the split
> extent should be 'ext4_ext_pblock(ex) + end - ee_block + 1'.
>
> This bug will cause xfstests shared/298 failure on ext4 with bigalloc
> enabled sometimes. Ext4 error messages indicate that previously freed
> blocks are being freed again, and the following fsck will fail due to
> the inconsistency of block bitmap and bg descriptor.
>
> The following is an example case:
>
> 1. First, Initialize a ext4 filesystem with cluster size '16K', block size
> '4K', in which case, one cluster contains four blocks.
>
> 2. Create one file (e.g., xxx.img) on this ext4 filesystem. Now the extent
> tree of this file is like:
>
> ...
> 36864:[0]4:220160
> 36868:[0]14332:145408
> 51200:[0]2:231424
> ...
>
> 3. Then execute PUNCH_HOLE fallocate on this file. The hole range is
> like:
>
> ..
> ext4_ext_remove_space: dev 254,16 ino 12 since 49506 end 49506 depth 1
> ext4_ext_remove_space: dev 254,16 ino 12 since 49544 end 49546 depth 1
> ext4_ext_remove_space: dev 254,16 ino 12 since 49605 end 49607 depth 1
> ...
>
> 4. Then the extent tree of this file after punching is like
>
> ...
> 49507:[0]37:158047
> 49547:[0]58:158087
> ...
>
> 5. Detailed procedure of punching hole [49544, 49546]
>
> 5.1. The block address space:
> ```
> lblk ~49505 49506 49507~49543 49544~49546 49547~
> ---------+------+-------------+----------------+--------
> extent | hole | extent | hole | extent
> ---------+------+-------------+----------------+--------
> pblk ~158045 158046 158047~158083 158084~158086 158087~
> ```
>
> 5.2. The detailed layout of cluster 39521:
> ```
> cluster 39521
> <------------------------------->
>
> hole extent
> <----------------------><--------
>
> lblk 49544 49545 49546 49547
> +-------+-------+-------+-------+
> | | | | |
> +-------+-------+-------+-------+
> pblk 158084 1580845 158086 158087
> ```
>
> 5.3. The ftrace output when punching hole [49544, 49546]:
> - ext4_ext_remove_space (start 49544, end 49546)
> - ext4_ext_rm_leaf (start 49544, end 49546, last_extent [49507(158047), 40], partial [pclu 39522 lblk 0 state 2])
> - ext4_remove_blocks (extent [49507(158047), 40], from 49544 to 49546, partial [pclu 39522 lblk 0 state 2]
> - ext4_free_blocks: (block 158084 count 4)
> - ext4_mballoc_free (extent 1/6753/1)
>
> In this case, the whole cluster 39521 is freed mistakenly when freeing
> pblock 158084~158086 (i.e., the first three blocks of this cluster),
> although pblock 158087 (the last remaining block of this cluster) has
> not been freed yet.
>
> The root cause of this isuue is that, the pclu of the partial cluster is
> calculated mistakenly in ext4_ext_remove_space(). The correct
> partial_cluster.pclu (i.e., the cluster number of the first block in the
> next extent, that is, lblock 49597 (pblock 158086)) should be 39521 rather
> than 39522.
>
> Fixes: f4226d9ea400 ("ext4: fix partial cluster initialization")
> Signed-off-by: Jeffle Xu <jefflexu@...ux.alibaba.com>
> ---
> fs/ext4/extents.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index f2b577b..cb74496 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -2828,7 +2828,7 @@ int ext4_ext_remove_space(struct inode *inode, ext4_lblk_t start,
> * in use to avoid freeing it when removing blocks.
> */
> if (sbi->s_cluster_ratio > 1) {
> - pblk = ext4_ext_pblock(ex) + end - ee_block + 2;
> + pblk = ext4_ext_pblock(ex) + end - ee_block + 1;
> partial.pclu = EXT4_B2C(sbi, pblk);
> partial.state = nofree;
> }
> --
> 1.8.3.1
>
Hi, Jeffle:
Thanks for the report and the patch. At first glance I suspect the "2" is
simply a bug; logically we're just looking for the first block after the
extent split to set the partial cluster, as you suggest. I'll post the
results of my review once I've had a chance to refresh my memory of the
code and run some more tests.
Thanks,
Eric
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