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[80.230.68.31]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-47aa0a558easm13544055f8f.27.2026.07.04.23.25.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 04 Jul 2026 23:25:15 -0700 (PDT) Date: Sun, 5 Jul 2026 02:25:13 -0400 From: "Michael S. Tsirkin" To: linux-kernel@vger.kernel.org Cc: "David Hildenbrand (Arm)" , Gregory Price , Zi Yan , Pankaj Gupta , Jason Wang , Xuan Zhuo , Eugenio =?utf-8?B?UMOpcmV6?= , Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Alexander Duyck , virtualization@lists.linux.dev, linux-mm@kvack.org Subject: [PATCH v3] mm: page_reporting: allow driver to set batch capacity Message-ID: <444c24cf39f3f3620fc90ef4695bd6b0979f4c4b.1783232420.git.mst@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline X-Mailer: git-send-email 2.51.2.2891.g4157995a80.dirty X-Mutt-Fcc: =sent 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. Fixes: b0c504f15471 ("virtio-balloon: add support for providing free page reports to host") Assisted-by: Claude:claude-opus-4-6 Acked-by: David Hildenbrand (Arm) Reviewed-by: Gregory Price Acked-by: Zi Yan Reviewed-by: Pankaj Gupta Signed-off-by: Michael S. Tsirkin --- Who's merging this? Me? Changes in v3: - Dropped stale "This value should always be a power of 2" comment above PAGE_REPORTING_CAPACITY (David Hildenbrand, others) Changes in v2: - Added paragraph explaining capacity need not be a power of two - Added paragraph documenting capacity=0 default and no div-by-zero (Gregory Price) - Improved struct field comment: "0 (default) means PAGE_REPORTING_CAPACITY" drivers/virtio/virtio_balloon.c | 5 +---- include/linux/page_reporting.h | 4 +++- mm/page_reporting.c | 24 ++++++++++++------------ 3 files changed, 16 insertions(+), 17 deletions(-) diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c index 088b3a0e6ce6..581ac799d974 100644 --- a/drivers/virtio/virtio_balloon.c +++ b/drivers/virtio/virtio_balloon.c @@ -1017,10 +1017,6 @@ static int virtballoon_probe(struct virtio_device *vdev) unsigned int capacity; capacity = virtqueue_get_vring_size(vb->reporting_vq); - if (capacity < PAGE_REPORTING_CAPACITY) { - err = -ENOSPC; - goto out_unregister_oom; - } vb->pr_dev_info.order = PAGE_REPORTING_ORDER_UNSPECIFIED; @@ -1041,6 +1037,7 @@ static int virtballoon_probe(struct virtio_device *vdev) vb->pr_dev_info.order = 5; #endif + vb->pr_dev_info.capacity = capacity; err = page_reporting_register(&vb->pr_dev_info); if (err) goto out_unregister_oom; 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 */ diff --git a/mm/page_reporting.c b/mm/page_reporting.c index 7418f2e500bb..942e84b6908a 100644 --- a/mm/page_reporting.c +++ b/mm/page_reporting.c @@ -173,11 +173,8 @@ page_reporting_cycle(struct page_reporting_dev_info *prdev, struct zone *zone, * any pages that may have already been present from the previous * list processed. This should result in us reporting all pages on * an idle system in about 30 seconds. - * - * The division here should be cheap since PAGE_REPORTING_CAPACITY - * should always be a power of 2. */ - budget = DIV_ROUND_UP(area->nr_free, PAGE_REPORTING_CAPACITY * 16); + budget = DIV_ROUND_UP(area->nr_free, prdev->capacity * 16); /* loop through free list adding unreported pages to sg list */ list_for_each_entry_safe(page, next, list, lru) { @@ -222,10 +219,10 @@ page_reporting_cycle(struct page_reporting_dev_info *prdev, struct zone *zone, spin_unlock_irq(&zone->lock); /* begin processing pages in local list */ - err = prdev->report(prdev, sgl, PAGE_REPORTING_CAPACITY); + err = prdev->report(prdev, sgl, prdev->capacity); /* reset offset since the full list was reported */ - *offset = PAGE_REPORTING_CAPACITY; + *offset = prdev->capacity; /* update budget to reflect call to report function */ budget--; @@ -234,7 +231,7 @@ page_reporting_cycle(struct page_reporting_dev_info *prdev, struct zone *zone, spin_lock_irq(&zone->lock); /* flush reported pages from the sg list */ - page_reporting_drain(prdev, sgl, PAGE_REPORTING_CAPACITY, !err); + page_reporting_drain(prdev, sgl, prdev->capacity, !err); /* * Reset next to first entry, the old next isn't valid @@ -260,13 +257,13 @@ static int page_reporting_process_zone(struct page_reporting_dev_info *prdev, struct scatterlist *sgl, struct zone *zone) { - unsigned int order, mt, leftover, offset = PAGE_REPORTING_CAPACITY; + unsigned int order, mt, leftover, offset = prdev->capacity; unsigned long watermark; int err = 0; /* Generate minimum watermark to be able to guarantee progress */ watermark = low_wmark_pages(zone) + - (PAGE_REPORTING_CAPACITY << page_reporting_order); + (prdev->capacity << page_reporting_order); /* * Cancel request if insufficient free memory or if we failed @@ -290,7 +287,7 @@ page_reporting_process_zone(struct page_reporting_dev_info *prdev, } /* report the leftover pages before going idle */ - leftover = PAGE_REPORTING_CAPACITY - offset; + leftover = prdev->capacity - offset; if (leftover) { sgl = &sgl[offset]; err = prdev->report(prdev, sgl, leftover); @@ -322,11 +319,11 @@ static void page_reporting_process(struct work_struct *work) atomic_set(&prdev->state, state); /* allocate scatterlist to store pages being reported on */ - sgl = kmalloc_objs(*sgl, PAGE_REPORTING_CAPACITY); + sgl = kmalloc_objs(*sgl, prdev->capacity); if (!sgl) goto err_out; - sg_init_table(sgl, PAGE_REPORTING_CAPACITY); + sg_init_table(sgl, prdev->capacity); for_each_zone(zone) { err = page_reporting_process_zone(prdev, sgl, zone); @@ -377,6 +374,9 @@ int page_reporting_register(struct page_reporting_dev_info *prdev) page_reporting_order = pageblock_order; } + if (!prdev->capacity || prdev->capacity > PAGE_REPORTING_CAPACITY) + prdev->capacity = PAGE_REPORTING_CAPACITY; + /* initialize state and work structures */ atomic_set(&prdev->state, PAGE_REPORTING_IDLE); INIT_DELAYED_WORK(&prdev->work, &page_reporting_process); -- MST