Re: Userspace filesystems (WAS: Encrypted Filesystem)

[Pavel Machek <pavel@ucw.cz>]

Hi!

> > >    app wants to read data from a file ->
> > >    userspace application requires memory allocation to provide this data ->
> > >    VM tries to write out dirty data associated with the Coda mountpoint ==
> > >    deadlock
> > 
> > How do you solve this one?
> 
> 1) In FUSE normal writes go through the cache, so no dirty pages are
>    created.  The only possibility to create dirty pages is with shared
>    writable mapping, and this is rare
> 
> 2) Userspace filesystem app can be multithreaded, so probably write
>    can be satisfied even if read is pending.
> 
> 3) The 2.6 kernel provides asynchronous page writeback, so even if a
>    writeback is blocking forever the VM will continue to try to free
>    up memory.
> 
> 4) If no memory can be freed, then the allocation will fail, so the
>    read will fail: no deadlock.

Transient real failure looks pretty ugly. I'd not expect
read(/etc/passwd) to return -ENOMEM, and read(/#ftp:somewhere/passwd)
should be the same, but as this is basically "can not happen"... I
guess that's enough.
								Pavel

-- 
When do you have a heart between your knees?
[Johanka's followup: and *two* hearts?]