Re: [RFC 0/3] mm: Discard lazily freed pages when migrating

Michal Hocko
2020-02-28T09:50:54Z

On Fri 28-02-20 16:55:40, Huang, Ying wrote:
> David Hildenbrand <david@redhat.com> writes:
[...]
> > E.g., free page reporting in QEMU wants to use MADV_FREE. The guest will
> > report currently free pages to the hypervisor, which will MADV_FREE the
> > reported memory. As long as there is no memory pressure, there is no
> > need to actually free the pages. Once the guest reuses such a page, it
> > could happen that there is still the old page and pulling in in a fresh
> > (zeroed) page can be avoided.
> >
> > AFAIKs, after your change, we would get more pages discarded from our
> > guest, resulting in more fresh (zeroed) pages having to be pulled in
> > when a guest touches a reported free page again. But OTOH, page
> > migration is speed up (avoiding to migrate these pages).
>
> Let's look at this problem in another perspective. To migrate the
> MADV_FREE pages of the QEMU process from the node A to the node B, we
> need to free the original pages in the node A, and (maybe) allocate the
> same number of pages in the node B. So the question becomes
>
> - we may need to allocate some pages in the node B
> - these pages may be accessed by the application or not
> - we should allocate all these pages in advance or allocate them lazily
> when they are accessed.
>
> We thought the common philosophy in Linux kernel is to allocate lazily.

The common philosophy is to cache as much as possible. And MADV_FREE
pages are a kind of cache as well. If the target node is short on memory
then those will be reclaimed as a cache so a pro-active freeing sounds
counter productive as you do not have any idea whether that cache is
going to be used in future. In other words you are not going to free a
clean page cache if you want to use that memory as a migration target
right? So you should make a clear case about why MADV_FREE cache is less
important than the clean page cache and ideally have a good
justification backed by real workloads.

--
Michal Hocko
SUSE Labs