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

H. Peter Anvin
2013-10-08T00:00:50Z

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.

-0hpa