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

John Stultz
2013-10-08T00:18:47Z

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.

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.

thanks
-john