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Message-ID: <60921135.3030900@huawei.com>
Date: Wed, 5 May 2021 11:29:57 +0800
From: yebin <yebin10@...wei.com>
To: Jan Kara <jack@...e.cz>
CC: <tytso@....edu>, <adilger.kernel@...ger.ca>,
<linux-ext4@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] ext4: Fix bug on in ext4_es_cache_extent as
ext4_split_extent_at failed
On 2021/4/30 20:58, Jan Kara wrote:
> On Wed 28-04-21 16:51:58, Ye Bin wrote:
>> We got follow bug_on when run fsstress with injecting IO fault:
>> [130747.323114] kernel BUG at fs/ext4/extents_status.c:762!
>> [130747.323117] Internal error: Oops - BUG: 0 [#1] SMP
>> ......
>> [130747.334329] Call trace:
>> [130747.334553] ext4_es_cache_extent+0x150/0x168 [ext4]
>> [130747.334975] ext4_cache_extents+0x64/0xe8 [ext4]
>> [130747.335368] ext4_find_extent+0x300/0x330 [ext4]
>> [130747.335759] ext4_ext_map_blocks+0x74/0x1178 [ext4]
>> [130747.336179] ext4_map_blocks+0x2f4/0x5f0 [ext4]
>> [130747.336567] ext4_mpage_readpages+0x4a8/0x7a8 [ext4]
>> [130747.336995] ext4_readpage+0x54/0x100 [ext4]
>> [130747.337359] generic_file_buffered_read+0x410/0xae8
>> [130747.337767] generic_file_read_iter+0x114/0x190
>> [130747.338152] ext4_file_read_iter+0x5c/0x140 [ext4]
>> [130747.338556] __vfs_read+0x11c/0x188
>> [130747.338851] vfs_read+0x94/0x150
>> [130747.339110] ksys_read+0x74/0xf0
>>
>> If call ext4_ext_insert_extent failed but new extent already inserted, we just
>> update "ex->ee_len = orig_ex.ee_len", this will lead to extent overlap, then
>> cause bug on when cache extent.
> Thanks for the patch but I'm still not quite sure, how overlapping extents
> in the extent tree can lead to triggering BUG_ON(lblk + len - 1 < lblk) in
> ext4_es_cache_extent(). Can you ellaborate a bit more how this happens?
Assume that there is extent [10, 100] (ee_block=10 ee_len=91), call
ext4_split_extent_at split at 50,
we get two extent [10, 49] and [50, 100], then call
ext4_ext_insert_extent to insert new extent [50, 100],
if insert extent successed, but call ext4_ext_dirty failed(return
-EROFS) as JBD maybe abort as io error.
Then fix old extent length with old value, so we get two extent
[10, 100] (ee_block=10 ee_len=91) and
[50, 100](ee_block=50 ee_len=51).
If call ext4_cache_extent to cache above extents as follow:
prev = 0 lblk = 10 len = 91 --> cache [10, 100] ---> prev = lblk + len
= 101
prev = 101 lblk = 50 len = 51 --> prev != 0 && prev != lblk --> cache
[prev = 101, lblk - prev = 50 - 101 = -51]
Obvious if call ext4_es_cache_extent cache extent[101, -51] wil trigger
"BUG_ON(end < lblk)" .
>> If call ext4_ext_insert_extent failed don't update ex->ee_len with old value.
>> Maybe there will lead to block leak, but it can be fixed by fsck later.
>>
>> After we fixed above issue with v2 patch, but we got the same issue.
>> ext4_split_extent_at:
>> {
>> ......
>> err = ext4_ext_insert_extent(handle, inode, ppath, &newex, flags);
>> if (err == -ENOSPC && (EXT4_EXT_MAY_ZEROOUT & split_flag)) {
>> ......
>> ext4_ext_try_to_merge(handle, inode, path, ex); ->step(1)
>> err = ext4_ext_dirty(handle, inode, path + path->p_depth); ->step(2)
>> if (err)
>> goto fix_extent_len;
>> ......
>> }
>> ......
>> fix_extent_len:
>> ex->ee_len = orig_ex.ee_len; ->step(3)
>> ......
>> }
>> If step(1) have been merged, but step(2) dirty extent failed, then go to
>> fix_extent_len label to fix ex->ee_len with orig_ex.ee_len. But "ex" may not be
>> old one, will cause overwritten. Then will trigger the same issue as previous.
>> If step(2) failed, just return error, don't fix ex->ee_len with old value.
>>
>> Signed-off-by: Ye Bin <yebin10@...wei.com>
>> ---
>> fs/ext4/extents.c | 13 +++++--------
>> 1 file changed, 5 insertions(+), 8 deletions(-)
>>
>> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
>> index 77c84d6f1af6..d4aa24a09d8b 100644
>> --- a/fs/ext4/extents.c
>> +++ b/fs/ext4/extents.c
>> @@ -3238,15 +3238,12 @@ static int ext4_split_extent_at(handle_t *handle,
>> ex->ee_len = cpu_to_le16(ee_len);
>> ext4_ext_try_to_merge(handle, inode, path, ex);
>> err = ext4_ext_dirty(handle, inode, path + path->p_depth);
>> - if (err)
>> - goto fix_extent_len;
>> -
>> - /* update extent status tree */
>> - err = ext4_zeroout_es(inode, &zero_ex);
>> -
>> - goto out;
>> - } else if (err)
>> + if (!err)
>> + /* update extent status tree */
>> + err = ext4_zeroout_es(inode, &zero_ex);
>> + } else if (err && err != -EROFS) {
> I fail to see why EROFS is special here. Can you explain a bit please?
V1 patch Ted suggest me to fix length only when "err != -EROSFS".
As if we don't
fix origin extent with old extent length, it will lead to block leak.
Ted said as follow:
If you don't want to do that, then a "do no harm" fix would be
something like this:
...
} else if (err == -EROFS) {
return err;
} else if (err)
goto fix_extent_len;
So in the journal abort case, when err is set to EROFS, we don't try
to reset the length, since in theory the file system is read-only
already anyway. However, in the ENOSPC case, we won't end up silently
leaking blocks that will be lost until the user somehow decides to run
fsck.
>> goto fix_extent_len;
>> + }
>>
>> out:
>> ext4_ext_show_leaf(inode, path);
> Honza
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