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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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