amd64/139806: Write attempt to file in ZFS snapshot dir causes panic

[Carl Chave]

>Number:         139806
>Category:       amd64
>Synopsis:       Write attempt to file in ZFS snapshot dir causes panic
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    freebsd-amd64
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Wed Oct 21 01:40:02 UTC 2009
>Closed-Date:
>Last-Modified:
>Originator:     Carl Chave
>Release:        8.0-RC1-amd64-dvd1.iso
>Organization:
>Environment:
8.0-RC1-amd64-dvd1.iso booted to Fixit environment in vmware.
>Description:
Attempting to modify a file in a zfs snapshot directory causes kernel panic with the following message:

panic: dirtying snapshot!

Maybe related to this PR:

http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/138764
>How-To-Repeat:
1. load zfs from bootloader prompt and then boot.
2. enter fixit environment.
3. import zpool (in this case a 2 disk vmware mirror named sodpool)
4. cd to previously created snapshot at sodpool/test/myfs/.zfs/snapshot/one
5. attempting to create a new file here results in:

Fixit# echo hello > hello.txt
cannot create hello.txt: Read-only file system

6. That seems like the desired response.  Next, attempt to modify a
file that already exists in the snapshot:

Fixit# echo hello >> test.txt
panic: dirtying snapshot!
>Fix:
Unknown

>Release-Note:
>Audit-Trail:
>Unformatted: