Re: [PATCH] fs: allow nesting with FREEZE_EXCL

[Christoph Hellwig]

On Fri, Apr 04, 2025 at 12:24:09PM +0200, Christian Brauner wrote:
> If hibernation races with filesystem freezing (e.g. DM reconfiguration),
> then hibernation need not freeze a filesystem because it's already
> frozen but userspace may thaw the filesystem before hibernation actually
> happens.
> 
> If the race happens the other way around, DM reconfiguration may
> unexpectedly fail with EBUSY.
> 
> So allow FREEZE_EXCL to nest with other holders. An exclusive freezer
> cannot be undone by any of the other concurrent freezers.

What is FREEZE_EXCL?  I can't find it anywhere including in linux-next.