Re: Unable to remove directory entry

Qu Wenruo
2019-12-09T01:53:09Z

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On 2019/12/9 上午9:33, Zygo Blaxell wrote:
> On Sun, Dec 08, 2019 at 07:17:21PM -0500, Zygo Blaxell wrote:
>> On Sun, Dec 08, 2019 at 02:19:10PM -0500, Mike Gilbert wrote:
>>> Hello,
>>>
>>> I have a directory entry that cannot be stat-ed or unlinked. This
>>> issue persists across reboots, so it seems there is something wrong on
>>> disk.
>>>
>>> % ls -l /var/cache/ccache.bad/2/c
>>> ls: cannot access
>>> '/var/cache/ccache.bad/2/c/0390cb341d248c589c419007da68b2-7351.manifest':
>>> No such
>>> file or directory
>>> total 0
>>> -????????? ? ? ? ? ? 0390cb341d248c589c419007da68b2-7351.manifest
>>
>> I have seen a bug similar to this some years ago. It was present as
>> far back as 4.5, and seems to still be present in 5.0.21. I don't have
>> detailed tracking information on it due to the low severity: not a crash
>> or data corruption bug, and workarounds exist both to prevent the bug
>> and to clean up its aftermath.
>>
>> The reproducer is something like:
>>
>> while (true) { // pseudocode
>> int fd = create(tmp_name);
>> write(fd, ...);
>> fsync(fd); // required, bug does not appear without this fsync
>> close(fd);
>> rename(tmp_name, regular_name);
>> }
>>
>> and a crash, maybe with some heavy write load. This is typical of
>> applications like git and ccache, and in the wild, broken directory
>> entries are often found in these applications' directories.
>>
>> Somewhere between 4.5 and 4.12 (a big range, I know), there was a change
>> in behavior: before, the broken directory entry could not be removed,
>> renamed, or used for a new file, the only way to get rid of the broken
>> directory entry was to delete the entire subvol. After the behavior
>> change, the broken directory entry could be removed by creating a new
>> file and renaming it to the broken directory entry name.
>
> I found a filesystem that currently has one of these broken dirents:
>
> root@tester24:/media/testfs/beeshome# ls -l
> ls: cannot access 'beesstats.txt.tmp': No such file or directory
> total 3446032
> -rw-r--r-- 1 root root 10313 Nov 22 2018 all-df-today.png
> -rw-r--r-- 1 root root 3297813 Nov 22 2018 all-df-today.txt
> -rw------- 1 root root 1048488 Dec 7 17:35 beescrawl.dat
> -rwx------ 1 root root 1073741824 Dec 8 20:19 beeshash.dat
> -????????? ? ? ? ? ? beesstats.txt.tmp
> -rw-r--r-- 1 root root 16064406 Dec 3 00:13 df-2019-11-28.txt
> -rw-r--r-- 1 root root 4269887 Dec 5 00:52 df-2019-12-03.txt
> -rw-r--r-- 1 root root 6358158 Dec 7 16:44 df-2019-12-05.txt
> -rw-r--r-- 1 root root 3221101 Dec 8 20:18 df-2019-12-07.txt
> -rw-r--r-- 1 root root 2208475574 Dec 3 00:13 log-2019-11-28.txt
> -rw-r--r-- 1 root root 72372394 Dec 5 00:52 log-2019-12-03.txt
> -rw-r--r-- 1 root root 97472346 Dec 7 16:44 log-2019-12-05.txt
> -rw-r--r-- 1 root root 42378425 Dec 8 20:19 log-2019-12-07.txt
> lrwxrwxrwx 1 root root 18 Dec 7 17:35 log-today.txt -> log-2019-12-07.txt
>
> It seems I can create a file with the same name, and then I get two:
>
> root@tester24:/media/testfs/beeshome# date > beesstats.txt.tmp
> root@tester24:/media/testfs/beeshome# ls -l
> total 3446044
> -rw-r--r-- 1 root root 10313 Nov 22 2018 all-df-today.png
> -rw-r--r-- 1 root root 3297813 Nov 22 2018 all-df-today.txt
> -rw------- 1 root root 1048488 Dec 7 17:35 beescrawl.dat
> -rwx------ 1 root root 1073741824 Dec 8 20:19 beeshash.dat
> -rw-r--r-- 1 root root 29 Dec 8 20:19 beesstats.txt.tmp
> -rw-r--r-- 1 root root 29 Dec 8 20:19 beesstats.txt.tmp
> -rw-r--r-- 1 root root 16064406 Dec 3 00:13 df-2019-11-28.txt
> -rw-r--r-- 1 root root 4269887 Dec 5 00:52 df-2019-12-03.txt
> -rw-r--r-- 1 root root 6358158 Dec 7 16:44 df-2019-12-05.txt
> -rw-r--r-- 1 root root 3221363 Dec 8 20:19 df-2019-12-07.txt
> -rw-r--r-- 1 root root 2208475574 Dec 3 00:13 log-2019-11-28.txt
> -rw-r--r-- 1 root root 72372394 Dec 5 00:52 log-2019-12-03.txt
> -rw-r--r-- 1 root root 97472346 Dec 7 16:44 log-2019-12-05.txt
> -rw-r--r-- 1 root root 42384027 Dec 8 20:19 log-2019-12-07.txt
> lrwxrwxrwx 1 root root 18 Dec 7 17:35 log-today.txt -> log-2019-12-07.txt
> root@tester24:/media/testfs/beeshome# cat beesstats.txt.tmp
> Sun Dec 8 20:19:38 EST 2019
>
> dump-tree sees both DIR_INDEX but only one DIR_ITEM:
>
> item 9 key (256 DIR_ITEM 2721875446) itemoff 15740 itemsize 47
> location key (133693 INODE_ITEM 0) type FILE
> transid 5002644 data_len 0 name_len 17
> name: beesstats.txt.tmp
> item 18 key (256 DIR_INDEX 22037) itemoff 15332 itemsize 47
> location key (11481 INODE_ITEM 0) type FILE
> transid 1876891 data_len 0 name_len 17
> name: beesstats.txt.tmp
> item 32 key (256 DIR_INDEX 264858) itemoff 14684 itemsize 47
> location key (133693 INODE_ITEM 0) type FILE
> transid 5002644 data_len 0 name_len 17
> name: beesstats.txt.tmp
>
> but I can only delete DIR_ITEMs:
>
> root@tester24:/media/testfs/beeshome# rm beesstats.txt.tmp
> root@tester24:/media/testfs/beeshome# rm beesstats.txt.tmp
> rm: cannot remove 'beesstats.txt.tmp': No such file or directory
> root@tester24:/media/testfs/beeshome# ls -l
> ls: cannot access 'beesstats.txt.tmp': No such file or directory
> total 3446048
> -rw-r--r-- 1 root root 10313 Nov 22 2018 all-df-today.png
> -rw-r--r-- 1 root root 3297813 Nov 22 2018 all-df-today.txt
> -rw------- 1 root root 1048488 Dec 7 17:35 beescrawl.dat
> -rwx------ 1 root root 1073741824 Dec 8 20:20 beeshash.dat
> -????????? ? ? ? ? ? beesstats.txt.tmp
> -rw-r--r-- 1 root root 16064406 Dec 3 00:13 df-2019-11-28.txt
> -rw-r--r-- 1 root root 4269887 Dec 5 00:52 df-2019-12-03.txt
> -rw-r--r-- 1 root root 6358158 Dec 7 16:44 df-2019-12-05.txt
> -rw-r--r-- 1 root root 3221494 Dec 8 20:19 df-2019-12-07.txt
> -rw-r--r-- 1 root root 2208475574 Dec 3 00:13 log-2019-11-28.txt
> -rw-r--r-- 1 root root 72372394 Dec 5 00:52 log-2019-12-03.txt
> -rw-r--r-- 1 root root 97472346 Dec 7 16:44 log-2019-12-05.txt
> -rw-r--r-- 1 root root 42396102 Dec 8 20:20 log-2019-12-07.txt
> lrwxrwxrwx 1 root root 18 Dec 7 17:35 log-today.txt -> log-2019-12-07.txt
>
> leaving the first DIR_INDEX behind:
>
> item 17 key (256 DIR_INDEX 22037) itemoff 15379 itemsize 47
> location key (11481 INODE_ITEM 0) type FILE
> transid 1876891 data_len 0 name_len 17
> name: beesstats.txt.tmp

This looks like a older kernel bug (hopes so).

So there is an orphan DIR_INDEX left, but never cleaned up properly.

In that case, btrfs-progs should be able to repair it.
But strangely, why original mode check didn't report it?

BTW, does that 11481 inode still exist?

Thanks,
Qu
>
> So the btrfs read side is fine, it's the writing side that is putting bad
> metadata on the disk.
>
>> Another workaround is to remove the fsync by running the application
>> under eatmydata. btrfs performs a flush in the rename() operation when
>> an existing file is replaced, so the fsync that triggers the bug was
>> not necessary in the first place. Note this only works when replacing
>> an existing file, so the flushoncommit mount option is required to make
>> this work in other cases.
>>
>>> % uname -a
>>> Linux naomi 4.19.67 #4 SMP Sun Aug 18 14:35:39 EDT 2019 x86_64 AMD
>>> Phenom(tm) II X6 1055T Processor
>>> AuthenticAMD GNU/Linux
>>>
>>> % btrfs --version
>>> btrfs-progs v5.4
>>>
>>> I have tried running btrfs check, and I get differing results based on
>>> the --mode switch:
>>>
>>> # btrfs check --readonly /dev/sda3
>>> [1/7] checking root items
>>> [2/7] checking extents
>>> [3/7] checking free space cache
>>> [4/7] checking fs roots
>>> [5/7] checking only csums items (without verifying data)
>>> [6/7] checking root refs
>>> [7/7] checking quota groups
>>> Opening filesystem to check...
>>> Checking filesystem on /dev/sda3
>>> UUID: 5e9dcab6-036d-40f1-8b40-24ab4c062bf6
>>> found 284337733632 bytes used, no error found
>>> total csum bytes: 267182280
>>> total tree bytes: 4498915328
>>> total fs tree bytes: 3972464640
>>> total extent tree bytes: 199819264
>>> btree space waste bytes: 776711635
>>> file data blocks allocated: 313928671232
>>> referenced 279141621760
>>>
>>> # btrfs check --readonly --mode=lowmem /dev/sda3
>>> [1/7] checking root items
>>> [2/7] checking extents
>>> [3/7] checking free space cache
>>> [4/7] checking fs roots
>>> ERROR: root 5 INODE_ITEM[4065004] index 18446744073709551615 name
>>> 0390cb341d248c589c419007da68b2-7351.manifest filetype 1 missing
>>> ERROR: root 5 DIR ITEM[486836 13905] name
>>> 0390cb341d248c589c419007da68b2-7351.manifest filetype 1 mismath
>>> ERROR: root 5 DIR ITEM[486836 2543451757] mismatch name
>>> 0390cb341d248c589c419007da68b2-7351.manifest filetype 1
>>> ERROR: errors found in fs roots
>>> Opening filesystem to check...
>>> Checking filesystem on /dev/sda3
>>> UUID: 5e9dcab6-036d-40f1-8b40-24ab4c062bf6
>>> found 284337733632 bytes used, error(s) found
>>> total csum bytes: 267182280
>>> total tree bytes: 4498915328
>>> total fs tree bytes: 3972464640
>>> total extent tree bytes: 199819264
>>> btree space waste bytes: 776711635
>>> file data blocks allocated: 313928671232
>>> referenced 279141621760
>>>
>>> Please advise on possible next steps to diagnose and fix this.
>
>

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