[Bug 217529] New: Remounting ext4 filesystem from ro to rw fails when quotas are enabled

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=217529

            Bug ID: 217529
           Summary: Remounting ext4 filesystem from ro to rw fails when
                    quotas are enabled
           Product: File System
           Version: 2.5
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P3
         Component: ext4
          Assignee: fs_ext4@kernel-bugs.osdl.org
          Reporter: nikolas.kraetzschmar@sap.com
        Regression: No

Description

Since commit a44be64, remounting a read-only ext4 filesystem to become
read-write fails when quotas are enabled. The mount syscall returns -EROFS and
outputs the following in dmesg:

```
EXT4-fs warning (device loop0): ext4_enable_quotas:7028: Failed to enable quota
tracking (type=0, err=-30, ino=3). Please run e2fsck
```


Root cause

The problem can be traced back to the changes introduced in commit a44be64. It
appears that the issue arises because the SB_RDONLY bit of the s_flags field is
now only cleared after executing the ext4_enable_quotas function. However, the
vfs_setup_quota_inode function, called by ext4_enable_quotas, checks whether
this bit is set (fs/quota/dquot.c:2331):

```
if (IS_RDONLY(inode))
        return -EROFS;
```

This condition therefore always triggers the -EROFS fail condition.


Steps to Reproduce

The bug can be reproduced by executing the following script on a current
mainline kernel with defconfig:

```
#!/bin/bash

set -ex

truncate -s 1G /tmp/img
mkfs.ext4 /tmp/img
tune2fs -Q usrquota,grpquota,prjquota /tmp/img
losetup /dev/loop0 /tmp/img
mount -o ro /dev/loop0 /mnt
mount -o remount,rw /mnt
```

Executing the script results in the following output:

```
+ truncate -s 1G /tmp/img
+ mkfs.ext4 /tmp/img
mke2fs 1.47.0 (5-Feb-2023)
Discarding device blocks: done
Creating filesystem with 262144 4k blocks and 65536 inodes
Filesystem UUID: b96a3da2-043f-11ee-b6f0-47c69db05231
Superblock backups stored on blocks:
        32768, 98304, 163840, 229376

Allocating group tables: done
Writing inode tables: done
Creating journal (8192 blocks): done
Writing superblocks and filesystem accounting information: done

+ tune2fs -Q usrquota,grpquota,prjquota /tmp/img
tune2fs 1.47.0 (5-Feb-2023)
+ losetup /dev/loop0 /tmp/img
[    6.766763] loop0: detected capacity change from 0 to 2097152
+ mount -o ro /dev/loop0 /mnt
[    6.791561] EXT4-fs (loop0): mounted filesystem
b96a3da2-043f-11ee-b6f0-47c69db05231 ro with ordered data mode. Quota mode:
journalled.
+ mount -o remount,rw /mnt
[    6.805546] EXT4-fs warning (device loop0): ext4_enable_quotas:7028: Failed
to enable quota tracking (type=0, err=-30, ino=3). Please run e2fsck to fix.
mount: /mnt: cannot remount /dev/loop0 read-write, is write-protected.
       dmesg(1) may have more information after failed mount system call.
```

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