K 10
svn:author
V 3
avg
K 8
svn:date
V 27
2018-03-27T14:31:42.047158Z
K 7
svn:log
V 737
vfs_donmount: in certain cases try r/o mount if r/w mount fails

If the operation is not an update, if neither r/w nor r/o mode is
explicitly requested, if the error code hints at the possibility of the
media being read-only, and if the fallback is allowed, then we can try
to automatically downgrade to the readonly mode.

This is especially useful for auto-mounting of removable media that
sometimes can happen to be write-protected.

The fallback to r/o is not enabled by default.  It can be requested on a
per-mount basis with a new mount option, 'autoro'.  Or it can be
globally allowed by setting vfs.default_autoro.

Reviewed by:	cem, kib
MFC after:	3 weeks
Relnotes:	yes
Differential Revision: https://reviews.freebsd.org/D13361

END