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    SubjectRe: [PATCH] Allow SysRq emergency sync to thaw frozen filesystems
    Andrew Morton wrote:
    > 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?

    Well, if you freeze root and do some things that require IO there, you
    can get stuck pretty easily

    (hacked xfs_io to call the ioctl here)

    [root@inode io]# ./xfs_io -r -x -F -c "freeze" /
    [root@inode io]# ls
    ^Z
    ^C
    <tap tap.. uhoh>

    >> 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().

    oh, hrm. Maybe I didn't think enough about how it's handed off to
    pdflush; I could rearrange if that makes sense? Or maybe handing to
    pdflush is wrong, it was just so convenient....

    > 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.

    Hm, maybe possible... I'll have to think about that.

    Thanks,
    -Eric

    >> ===================================================================
    >> --- 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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