Re: [PATCH v2] exfat: clear the volume dirty flag only when remounting read-only

[Namjae Jeon]

On Fri, Sep 4, 2026 at 12:27 PM Chi Zhiling <chizhiling@xxxxxxx> wrote:
>
> From: Chi Zhiling <chizhiling@xxxxxxxxxx>
>
> exfat_reconfigure() does not hold s_lock while calling
> sync_filesystem(). Therefore, the filesystem can become dirty again
> between the sync and clearing the volume dirty flag, leaving a dirty
> filesystem with a clean volume flag.
>
> Holding s_lock across sync_filesystem() is not an option because it can
> lead to a deadlock: writeback takes s_lock in exfat_write_inode().
> Instead, clear the volume dirty flag only when remounting the filesystem
> read-only, where no writer can modify the filesystem after the sync.
>
> For a normal read-only remount, reconfigure_super() calls
> sb_prepare_remount_readonly() before ->reconfigure(), which returns
> -EBUSY if any writer is still active. Afterwards
> sb_start_ro_state_change() sets sb->s_readonly_remount, so
> mnt_get_write_access() fails with -EROFS and no new writer can start
> until the reconfiguration finishes. Therefore no writer can race with
> the sync and the clearing.
>
> Forced remounts are different: with SB_FORCE, reconfigure_super() skips
> sb_prepare_remount_readonly(), so the writers active at that moment are
> not blocked and can still dirty the filesystem after the sync. Do not
> clear the volume dirty flag in this case.
>
> Signed-off-by: Chi Zhiling <chizhiling@xxxxxxxxxx>
Applied it to #dev.
Thanks!