Fwd: Remounting ext4 filesystem from ro to rw fails when quotas are enabled

[Bagas Sanjaya <bagasdotme@gmail.com>]

Hi,

I notice a regression report on Bugzilla [1]. Quoting from it:

> 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.
> ```

See Bugzilla for the full thread.

Ted, it looks like this regression is caused by your ext4_xattr_block_set()
fix to earlier syzbot report. Would you like to take a look on it?

Anyway, I'm adding it to regzbot:

#regzbot introduced: a44be64bbecb15 https://bugzilla.kernel.org/show_bug.cgi?id=217529
#regzbot title: Remounting ext4 filesystem from ro to rw fails when quotas are enabled

Thanks.

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

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