From 47c81dadb704cdf07bb8f9f43b4fbe758a351e08 Mon Sep 17 00:00:00 2001 From: "Michael S. Tsirkin" Date: Sun, 5 Jul 2026 02:25:13 -0400 Subject: mm: page_reporting: allow driver to set batch capacity MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Assisted-by: Claude:claude-opus-4-6 Acked-by: David Hildenbrand (Arm) Reviewed-by: Gregory Price Acked-by: Zi Yan Reviewed-by: Pankaj Gupta Cc: Alexander Duyck Cc: Brendan Jackman Cc: Eugenio Pérez Cc: Jason Wang Cc: Johannes Weiner Cc: Liam R. Howlett Cc: Lorenzo Stoakes Cc: Michal Hocko Cc: Mike Rapoport Cc: Suren Baghdasaryan Cc: Vlastimil Babka Cc: Xuan Zhuo Signed-off-by: Andrew Morton --- include/linux/page_reporting.h | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) (limited to 'include') 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 #include -/* 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 */ -- cgit v1.2.3