mm: page_reporting: allow driver to set batch capacity

At the moment, if a virtio balloon device has a page reporting vq but its
size is < PAGE_REPORTING_CAPACITY (32), the balloon driver fails probe.

But, there's no way for host to know this value, so it can easily create a
smaller vq and suddenly adding the reporting capability to the device
makes all of the driver fail.  Not pretty.

Add a capacity field to page_reporting_dev_info so drivers can control the
maximum number of pages per report batch.

In virtio-balloon, set the capacity to the reporting virtqueue size,
letting page_reporting adapt to whatever the device provides.

Capacity need not be a power of two.  Code previously called out division
by PAGE_REPORTING_CAPACITY as cheap since it was a power of 2, but no
performance difference was observed with non-power-of-2 values.

If capacity is 0 or exceeds PAGE_REPORTING_CAPACITY, it defaults to
PAGE_REPORTING_CAPACITY.  The 0 check and the clamping is done in
page_reporting_register(), before the reporting work is scheduled, so we
never get division by 0.

Link: https://lore.kernel.org/444c24cf39f3f3620fc90ef4695bd6b0979f4c4b.1783232420.git.mst@redhat.com
Fixes: b0c504f15471 ("virtio-balloon: add support for providing free page reports to host")
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Assisted-by: Claude:claude-opus-4-6
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Gregory Price <gourry@gourry.net>
Acked-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Eugenio Pérez <eperezma@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 3 insertions, 1 deletions

diff --git a/include/linux/page_reporting.h b/include/linux/page_reporting.h
index 9d4ca5c218a0..272b1274efdc 100644
--- a/include/linux/page_reporting.h
+++ b/include/linux/page_reporting.h
@@ -5,7 +5,6 @@
 #include <linux/mmzone.h>
 #include <linux/scatterlist.h>
 
-/* This value should always be a power of 2, see page_reporting_cycle() */
 #define PAGE_REPORTING_CAPACITY		32
 #define PAGE_REPORTING_ORDER_UNSPECIFIED	-1
 
@@ -22,6 +21,9 @@ struct page_reporting_dev_info {
 
 	/* Minimal order of page reporting */
 	unsigned int order;
+
+	/* Max pages per report batch; 0 (default) means PAGE_REPORTING_CAPACITY */
+	unsigned int capacity;
 };
 
 /* Tear-down and bring-up for page reporting devices */