> On Tuesday 18 September 2007 02:58, Peter Zijlstra wrote:
> > On Mon, 17 Sep 2007 22:11:25 -0700 Daniel Phillips wrote:
> > > > I've been using Avi Kivity's patch from some time ago:
> > > >
http://lkml.org/lkml/2004/7/26/68
> > >
> > > Yes. Ddsnap includes a bit of code almost identical to that, which
> > > we wrote independently. Seems wild and crazy at first blush,
> > > doesn't it? But this approach has proved robust in practice, and is
> > > to my mind, obviously correct.
> >
> > I'm so not liking this :-(
>
> Why don't you share your specific concerns?
>
> > Can't we just run the user-space part as mlockall and extend netlink
> > to work with PF_MEMALLOC where needed?
> >
> > I did something like that for iSCSI.
>
> Not sure what you mean by extend netlink. We do run the user daemons
> under mlockall of course, this is one of the rules I stated earlier for
> daemons running in the block IO path. The problem is, if this
> userspace daemon allocates even one page, for example in sys_open, it
> can deadlock. Running the daemon in PF_MEMALLOC mode fixes this
> problem robustly, provided that the necessary audit of memory
> allocation patterns and library dependencies has been done.
>
> I suppose you are worried that the userspace code could unexpectedly
> allocate a large amount of memory and exhaust the entire PF_MEMALLOC
> reserve? Kernel code could do that too. This userspace code just
> needs to be checked carefully. Perhaps we could come up with a kernel
> debugging option to verify that a task does in fact stay within some
> bounded number of page allocs while in PF_MEMALLOC mode.
