Re: [PATCH] [Request for inclusion] Filesystem in Userspace

[Elladan <elladan@eskimo.com>]

On Thu, Nov 18, 2004 at 10:31:27AM -0800, Linus Torvalds wrote:
> 
> 
> On Thu, 18 Nov 2004, Miklos Szeredi wrote:
> > 
> > Well, killing the fuse process _will_ make the system come back to
> > life, since then all the dirty pages belonging to the filesystem will
> > be discarded. 
> 
> They will? Why? They're still mapped into other processes, still dirty. 
> How do they go away?

If the filesystem a dirty page is mapped against ceases to exist,
wouldn't it make sense to destroy the page?  All such user processes can
receive SIGBUS and crash.  This is sort of a general problem with a
filesystem on a "removable" device like a USB stick, and it seems the
only sane solution is to blow all the mappings against that filesystem
away.

Of course, it sucks since the final result will be a crash, but mmap()
isn't a reliable interface for accessing media that might go away...

So, a reasonable solution would be to detect when a user fs process
dies, and initiate a forced unmount procedure where you walk all the
pages mapped against that filesystem and blow them away.  Similarly for
a case like pulling a USB reader out while it's being written to (you
could nag the user to reinsert, but that might be impossible).

This doesn't solve the deadlock as such except as a sort of
panic-recovery hack, but it seems sensible in general.

-J