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SubjectRe: [PATCH] Allow SysRq emergency sync to thaw frozen filesystems
On Wed, 14 Jan 2009 22:06:17 -0600
Eric Sandeen <sandeen@redhat.com> wrote:

> Now that the filesystem freeze operation has been elevated
> to the VFS, and is just an ioctl away, some sort of safety net
> for unintentionally frozen root filesystems may be in order.
>
> The timeout thaw originally proposed did not get merged, but
> perhaps something like this would be useful in emergencies.
>
> This doesn't have to piggyback on the existing emergency sync
> sysrq, but it seems like a reasonable, simple addition to me.
>
> I've tested this on a non-root fs with multiple (nested) freezers,
> as well as on a system rendered unresponsive due to a frozen
> root fs.

Worried.

Under what operational scenarios is ths feature actually needed/used?

> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
> ---
>
> Index: linux-2.6/drivers/char/sysrq.c
> ===================================================================
> --- linux-2.6.orig/drivers/char/sysrq.c
> +++ linux-2.6/drivers/char/sysrq.c
> @@ -151,6 +151,7 @@ static struct sysrq_key_op sysrq_reboot_
>
> static void sysrq_handle_sync(int key, struct tty_struct *tty)
> {
> + emergency_thaw();
> emergency_sync();
> }

Kind of weird. The thaw will happen after/during the sync().

I guess that if the sync is blocked on a frozen fs then things will
sort themselves out.

otoh, if all the pdflush threads are blocked on frozen filesystems
(possible?) then the emergency_thaw() simply won't do anything.

> ===================================================================
> --- linux-2.6.orig/fs/buffer.c
> +++ linux-2.6/fs/buffer.c
> @@ -258,6 +258,29 @@ struct super_block *freeze_bdev(struct b
> }
> EXPORT_SYMBOL(freeze_bdev);
>
> +void do_thaw(unsigned long unused)
> +{
> + struct super_block *sb;
> + char b[BDEVNAME_SIZE];
> +
> + list_for_each_entry(sb, &super_blocks, s_list) {
> + while (sb->s_bdev && !thaw_bdev(sb->s_bdev, sb))
> + printk(KERN_WARNING "Emergency Thaw on %s\n",
> + bdevname(sb->s_bdev, b));

hm, I made the args to bdevname() backwards. Bad me.

> + }
> + printk(KERN_WARNING "Emergency Thaw complete\n");
> +}



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