>>> This one could be solved by deleting the file on disk when the
>>> filesystem is made read-only, I think.
>> That will [...] not, however, preserve the long-standing Unix
>> semantics of continuing to use open-but-unlinked files.
> Why couldn't the file continue to be used, even if it is deleted on
> disk?  Remember, the filesystem is now RO, so nothing can reuse the
> freed data and metadata for another purpose.

This depends on the filesystem, of course.

For FFS, I think this would work: the inode is cleared, but the inode
is locked in core anyway while the file is open - and the data and
indirect blocks will be marked free but, as you say, their contents
will be undisturbed.

I don't know other filesystems enough to speak to them.

However, even for FFS, it will break if you switch from RW to RO and
then back to RW.

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