Re: [PATCH 2/3] exec: posix_madvise usage on SunOS.

[Peter Maydell]

On Tue, 21 Jul 2020 at 09:22, David Hildenbrand <david@redhat.com> wrote:
> virtio-mem depends on Linux (hw/virtio/Kconfig). I guess
> userfaultfd/postcopy is also not relevant in the context of SunOS. So
> what remains is virtio-balloon.
>
> virito-balloon ideally wants to discard the actual mapped pages to free
> up memory. When memory is re-accessed, a fresh page is faulted in (->
> zero-page under Linux). Now, we already have other cases where it looks
> like "the balloon works" but it really doesn't. One example is using
> vfio+virtio-balloon under Linux - inflating the balloon is simply a NOP,
> no memory is actually discarded.
>
> I agree that POSIX_MADV_DONTNEED is not a proper match - different
> guarantees. If SunOS cannot implement ram_block_discard_range() as
> documented, we should disable it.

Could we also improve the documentation comment to make it clearer
what ram_block_discard_range() is actually supposed to be doing?
At the moment I don't think you can figure it out from the comments,
which are a confusing mix of claiming it unmaps memory and that it
zeroes it.

Is the Linux-specific stuff (userfaultfd) a *requirement* for the
function, or just a "this would be nice" extra and a valid
implementation would be eg "zero out the memory" ?

thanks
-- PMM