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>

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 088b3a0e6ce6cc776ea144edd53fa73dc785bb91..581ac799d97495114a79872a3b3bee3d2b65e83d 100644 (file)
--- 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 9d4ca5c218a0fb5bc9e07e202524ce7613a200c8..272b1274efdc1c18dc5117e69924e1181338b2af 100644 (file)
--- 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 */

diff --git a/mm/page_reporting.c b/mm/page_reporting.c
index 7418f2e500bb481dd6c64466cc140f4a0283ac70..942e84b6908af75ff75ed400126c76b2c3962109 100644 (file)
--- 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);