Re: [PATCH] Syscalls: reboot: Add options to the reboot syscall to remount filesystems ro

[Ingo Molnar <mingo@elte.hu>]

* Ken Sumrall <ksumrall@android.com> wrote:

> Add 4 new commands to the reboot system call, that do the same thing as the
> RESTART, HALT, POWER_OFF, and RESTART2 commands, but also remount writable
> filesystems as read-only just before doing what the command normally does.
> Now that Android is using EXT4, and since we don't have a standard init
> setup to unmount filesystems before rebooting, this allows the system to
> reboot with clean filesystems, and also improves boot time as the journal
> does not need to be replayed when mounting the filesystem.
> 
> Signed-off-by: Ken Sumrall <ksumrall@android.com>
> ---
>  fs/super.c             |    9 +++++++++
>  include/linux/fs.h     |    1 +
>  include/linux/reboot.h |    4 ++++
>  kernel/sys.c           |   12 ++++++++++++
>  4 files changed, 26 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/super.c b/fs/super.c
> index 8819e3a..3f39a16 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -638,6 +638,15 @@ void emergency_remount(void)
>  	}
>  }
>  
> +void emergency_remount_synchronous(void)
> +{
> +	struct work_struct *work;
> +
> +	work = kmalloc(sizeof(*work), GFP_ATOMIC);
> +	if (work)
> +		do_emergency_remount(work);
> +}
> +
>  /*
>   * Unnamed block devices are dummy devices used by virtual
>   * filesystems which don't use real block-devices.  -- jrs
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 63d069b..e48ef0d 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2110,6 +2110,7 @@ extern int generic_write_sync(struct file *file, loff_t pos, loff_t count);
>  extern void sync_supers(void);
>  extern void emergency_sync(void);
>  extern void emergency_remount(void);
> +extern void emergency_remount_synchronous(void);
>  #ifdef CONFIG_BLOCK
>  extern sector_t bmap(struct inode *, sector_t);
>  #endif
> diff --git a/include/linux/reboot.h b/include/linux/reboot.h
> index 3005d5a..24b185d 100644
> --- a/include/linux/reboot.h
> +++ b/include/linux/reboot.h
> @@ -26,11 +26,15 @@
>   */
>  
>  #define	LINUX_REBOOT_CMD_RESTART	0x01234567
> +#define	LINUX_REBOOT_CMD_RMNT_RESTART	0x12345670
>  #define	LINUX_REBOOT_CMD_HALT		0xCDEF0123
> +#define	LINUX_REBOOT_CMD_RMNT_HALT	0xDEF0123C
>  #define	LINUX_REBOOT_CMD_CAD_ON		0x89ABCDEF
>  #define	LINUX_REBOOT_CMD_CAD_OFF	0x00000000
>  #define	LINUX_REBOOT_CMD_POWER_OFF	0x4321FEDC
> +#define	LINUX_REBOOT_CMD_RMNT_POWER_OFF	0x321FEDC4
>  #define	LINUX_REBOOT_CMD_RESTART2	0xA1B2C3D4
> +#define	LINUX_REBOOT_CMD_RMNT_RESTART2	0x1B2C3D4A
>  #define	LINUX_REBOOT_CMD_SW_SUSPEND	0xD000FCE2
>  #define	LINUX_REBOOT_CMD_KEXEC		0x45584543
>  
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 7f5a0cd..3f474e6 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -392,6 +392,9 @@ SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,
>  	mutex_lock(&reboot_mutex);
>  	switch (cmd) {
>  	case LINUX_REBOOT_CMD_RESTART:
> +	case LINUX_REBOOT_CMD_RMNT_RESTART:
> +		if (cmd == LINUX_REBOOT_CMD_RMNT_RESTART)
> +			emergency_remount_synchronous();
>  		kernel_restart(NULL);
>  		break;
>  
> @@ -404,22 +407,31 @@ SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,
>  		break;
>  
>  	case LINUX_REBOOT_CMD_HALT:
> +	case LINUX_REBOOT_CMD_RMNT_HALT:
> +		if (cmd == LINUX_REBOOT_CMD_RMNT_HALT)
> +			emergency_remount_synchronous();
>  		kernel_halt();
>  		do_exit(0);
>  		panic("cannot halt");
>  
>  	case LINUX_REBOOT_CMD_POWER_OFF:
> +	case LINUX_REBOOT_CMD_RMNT_POWER_OFF:
> +		if (cmd == LINUX_REBOOT_CMD_RMNT_POWER_OFF)
> +			emergency_remount_synchronous();
>  		kernel_power_off();
>  		do_exit(0);
>  		break;
>  
>  	case LINUX_REBOOT_CMD_RESTART2:
> +	case LINUX_REBOOT_CMD_RMNT_RESTART2:
>  		if (strncpy_from_user(&buffer[0], arg, sizeof(buffer) - 1) < 0) {
>  			ret = -EFAULT;
>  			break;
>  		}
>  		buffer[sizeof(buffer) - 1] = '\0';
>  
> +		if (cmd == LINUX_REBOOT_CMD_RMNT_RESTART2)
> +			emergency_remount_synchronous();
>  		kernel_restart(buffer);
>  		break;

Wouldnt it be *much* simpler to add it as a magic1 variant:

 #define LINUX_REBOOT_UMOUNT_RO   0xfee1deaf

Used as a 'also remount ro please' flag?

That way the whole patch would literally be 2 lines:

	if (magic1 == LINUX_REBOOT_UMOUNT_RO)
		emergency_remount_synchronous();

Note that this also has the advantage that both kexec and suspend reboots could be 
done with emergency umounts, if so desired - and any future reboot variant would be 
supported as well. Your patch left out those other reboot methods.

Thanks,

	Ingo