Re: [RFC] page fault retry with NOPAGE_RETRY

Benjamin Herrenschmidt
2006-09-20T00:06:46Z

On Tue, 2006-09-19 at 16:59 -0700, Andrew Morton wrote:
> On Wed, 20 Sep 2006 09:50:35 +1000
> Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:
>
> > On Tue, 2006-09-19 at 16:35 -0700, Mike Waychison wrote:
> > > Patch attached.
> > >
> > > As Andrew points out, the logic is a bit hacky and using a flag in
> > > current->flags to determine whether we have done the retry or not already.
> > >
> > > I too think the right approach to being able to handle these kinds of
> > > retries in a more general fashion is to introduce a struct
> > > pagefault_args along the page faulting path. Within it, we could
> > > introduce a reason for the retry so the higher levels would be able to
> > > better understand what to do.
> >
> > .../...
> >
> > I need to re-read your mail and Andrew as at this point, I don't quite
> > see why we need that args and/or that current->flags bit instead of
> > always returning all the way to userland and let the faulting
> > instruction happen again (which means you don't block in the kernel, can
> > take signals etc...
>
> That would amount to a busy wait, waiting for the disk IO to complete.
>
> So we need to go to sleep somewhere (in D state, because we _are_ waiting
> for disk IO). Returning all the way to userspace and immediately retaking
> the fault is unneeded extra work.

No, I'm not saying immediately... you do the wait thing in filemap.c.
Anyway, see my other message.

> > thus do you actually need to prevent multiple
> > retries ?)
>
> I expect there are livelock scenarios. For example, process A could spin
> on posix_fadvise(some libc text page, POSIX_FADV_DONTNEED), perhaps causing
> other applications to get permanently stuck in the kernel.

Unless you add a way to handle signals.. see my other mail.

Ben.