Re: [PATCH 05/14] vrange: Add new vrange(2) system call

Minchan Kim
2013-10-08T00:33:15Z

On Mon, Oct 07, 2013 at 05:18:40PM -0700, John Stultz wrote:
> On 10/07/2013 05:13 PM, Minchan Kim wrote:
> > Hello Peter,
> >
> > On Mon, Oct 07, 2013 at 04:59:40PM -0700, H. Peter Anvin wrote:
> >> On 10/07/2013 04:54 PM, John Stultz wrote:
> >>>> And wouldn't this apply to MADV_DONTNEED just as well? Perhaps what we
> >>>> should do is an enhanced madvise() call?
> >>> Well, I think MADV_DONTNEED doesn't *have* do to anything at all. Its
> >>> advisory after all. So it may immediately wipe out any data, but it may not.
> >>>
> >>> Those advisory semantics work fine w/ VRANGE_VOLATILE. However,
> >>> VRANGE_NONVOLATILE is not quite advisory, its telling the system that it
> >>> requires the memory at the specified range to not be volatile, and we
> >>> need to correctly inform userland how much was changed and if any of the
> >>> memory we did change to non-volatile was purged since being set volatile.
> >>>
> >>> In that way it is sort of different from madvise. Some sort of an
> >>> madvise2 could be done, but then the extra purge state argument would be
> >>> oddly defined for any other mode.
> >>>
> >>> Is your main concern here just wanting to have a zero-fill mode with
> >>> volatile ranges? Or do you really want to squeeze this in to the madvise
> >>> call interface?
> >> The point is that MADV_DONTNEED is very similar in that sense,
> >> especially if allowed to be lazy. It makes a lot of sense to permit
> >> both scrubbing modes orthogonally.
> >>
> >> The point you're making has to do with withdrawal of permission to flush
> >> on demand, which is a result of having the lazy mode (ongoing
> >> permission) and having to be able to withdraw such permission.
> > I'm sorry I could not understand what you wanted to say.
> > Could you elaborate a bit?
> My understanding of his point is that VRANGE_VOLATILE is like a lazy
> MADV_DONTNEED (with sigbus, rather then zero fill on fault), suggests
> that we should find a way to have VRANGE_VOLATILE be something like
> MADV_DONTNEED|MADV_LAZY|MADV_SIGBUS_FAULT, instead of adding a new
> syscall. This would provide more options, since one could instead just
> do MADV_DONTNEED|MADV_LAZY if they wanted zero-fill faults.

Hmm, actually, I have thought VRANGE_SIGBUS option because Address/Thread
sanitizer people wanted it as you know and someone might want it, too.

I agree it's orthogonal but not sure MADV_LAZY and MADV_SIGBUS_FAULT can be
used for other combination of advise except MADV_DONTNEED so it might
confuse userland without benefit.

>
> And indeed, for the VRANGE_VOLATILE case, we could do something like
> that, but the unresolved problem I see is that that we still need to
> handle the VRANGE_NONVOLATILE case, and the madvise() interface doesn't
> seem to accomodate the needed semantics well.

VRANGE_VOLATILE case could be a problem. In my mind, I had an idea to
return purged state when we call vrange(VRANGE_VOLATILE) because kernel
could purge them as soon as vrange(VRANGE_VOLATILE) called if memory is
really tight so userland can notice "purging" earlier and kernel can
discard them more efficiently.

>
> thanks
> -john
>
>
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--
Kind regards,
Minchan Kim