On 1/19/26 17:30, Thomson, Jack wrote:
On 19/01/2026 3:50 pm, David Hildenbrand (Red Hat) wrote:
On 1/19/26 16:42, Jack Thomson wrote:
From: Jack Thomson <[email protected]>
This RFC patch adds a new virtio feature for the virtio-balloon driver
during free page hinting, which will wait on device ack before
committing the range to the free_page_list. The reason for the change is
it allows the device to modify this range without it being reclaimed
from the free_page_list before the ack is sent. As expected, testing
shows this adds overhead to the hinting run duration, increasing it by
~30% with our Firecracker setup. Currently free page hinting is used
mainly for live migration, but this would open it up for a new use-case.
We would like to leverage this with MADV_DONTNEED to reduce RSS of a
guest. We'd like to use hinting because of the flexibility of control it
brings compared to reporting, allowing memory to be reclaimed in
deterministic periods.
Can you elaborate in more detail why you don't simply use reporting,
like QEMU?
Ideally we'd like to use hinting as the API allows us to control when
this reclamation takes place so as not to impact active VMs. For example
if we know a VM is idle we can reclaim memory but also cancel the
reclamation quickly if the VM receives new work (something we can't do
quickly with the traditional balloon.)
Could you instead see optimizations being done to reporting that could
make it fly for your use case?
One thing that I considered was having reporting running but skip
reported ranges during active times. But this may lead to missing
reclamation opportunities.
We could implement a pause+continue option for free-page-reporting
option. So the device could tell the VM to pause reporting and later to
restart reporting.
Hinting is a rather special case thing only used for reducing VM
migration time in QEMU AFAIR.
Yeah, its API allowing direct control was what interested us. With this
extension it made a great pairing just needed the synchronisation to
make it safe.
Sorry for getting back to you only now.
So, right now the thing is that hinted pages can get reused by the VM
any time. The hypervisor must detect if that happened and not discard
the pages in that case.
While that works for live migration with bitmap dirty tracking (and is a
bit confusing ...), it doesn't work when you want to MADV_DONTNEED that
memory, because it could be the hypervisor issues MADV_DONTNEED just
after the VM reused the memory.
So you are proposing to let the VM wait for the ack before possibly
reusing the pages.
One thing to note is that free page hinting in Linux allocates memory
through
alloc_pages(VIRTIO_BALLOON_FREE_PAGE_ALLOC_FLAG,
VIRTIO_BALLOON_HINT_BLOCK_ORDER);
Meaning
a) Limited to MAX_ORDER chunks (e.g., 4MB on x86). This is even bigger
than the free-page-reporting granularity (pageblock order, 2MB on
x86)
b) Cannot free memory on ZONE_MOVABLE or CMA in the VM (as these are
unmovable allocations)
So it's a bit suboptimal.
Also, in contrast to free-page-reporting, these pages are not going to
get reused unless we run into the shrinker, which is a bit suboptimal as
well. Free-page-reporting is a lot more optimized for that, as it just
returns reported pages back to the buddy immediately.
So if possible, I would suggest instead to extend free-page-reporting.
--
Cheers,
David
