[Lguest] proposed patch for mmap'ed private block device

[Rusty Russell]

On Thursday 22 May 2008 06:35:12 ron minnich wrote:
> This is another proposed patch for block devices to have them use mmap
> instead of file io.
>
> I use this several ways:
> 1. regression testing. I can boot a guest, have it mess around,
> literally rm -rf all of / with no harm done
> 2. lots of guests. This saves overhead as lots of mmap'ed pages are shared

OK, patch is nice.  It only works for small disks unfortunately, but it's 
simpler than using a (sparse) temporary file and a bitmap (as I did in qemu).

I was thinking of something else: if we added an ioctl to throw away (parts 
of) page tables, you can mmap (assuming alignments work) reads directly into 
the guest.  ISTR Anthony Liguori speculating on something similar.

> 3. lockups. I'm getting a deadlock or lockup on when I have lots of
> guests doing block io; I want to see if this fixes it.

This seems really odd.  I haven't stress tested here, but what are the 
symptoms?

Thanks,
Rusty.