linux-kernel - [PATCH v2 5/7] virtio-mem: try to merge system ram resources

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Date:   Tue,  8 Sep 2020 22:10:10 +0200
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     virtualization@...ts.linux-foundation.org, linux-mm@...ck.org,
        linux-hyperv@...r.kernel.org, xen-devel@...ts.xenproject.org,
        linux-acpi@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-s390@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Pankaj Gupta <pankaj.gupta.linux@...il.com>,
        Baoquan He <bhe@...hat.com>,
        Wei Yang <richardw.yang@...ux.intel.com>
Subject: [PATCH v2 5/7] virtio-mem: try to merge system ram resources

virtio-mem adds memory in memory block granularity, to be able to
remove it in the same granularity again later, and to grow slowly on
demand. This, however, results in quite a lot of resources when
adding a lot of memory. Resources are effectively stored in a list-based
tree. Having a lot of resources not only wastes memory, it also makes
traversing that tree more expensive, and makes /proc/iomem explode in
size (e.g., requiring kexec-tools to manually merge resources later
when e.g., trying to create a kdump header).

Before this patch, we get (/proc/iomem) when hotplugging 2G via virtio-mem
on x86-64:
        [...]
        100000000-13fffffff : System RAM
        140000000-33fffffff : virtio0
          140000000-147ffffff : System RAM (virtio_mem)
          148000000-14fffffff : System RAM (virtio_mem)
          150000000-157ffffff : System RAM (virtio_mem)
          158000000-15fffffff : System RAM (virtio_mem)
          160000000-167ffffff : System RAM (virtio_mem)
          168000000-16fffffff : System RAM (virtio_mem)
          170000000-177ffffff : System RAM (virtio_mem)
          178000000-17fffffff : System RAM (virtio_mem)
          180000000-187ffffff : System RAM (virtio_mem)
          188000000-18fffffff : System RAM (virtio_mem)
          190000000-197ffffff : System RAM (virtio_mem)
          198000000-19fffffff : System RAM (virtio_mem)
          1a0000000-1a7ffffff : System RAM (virtio_mem)
          1a8000000-1afffffff : System RAM (virtio_mem)
          1b0000000-1b7ffffff : System RAM (virtio_mem)
          1b8000000-1bfffffff : System RAM (virtio_mem)
        3280000000-32ffffffff : PCI Bus 0000:00

With this patch, we get (/proc/iomem):
        [...]
        fffc0000-ffffffff : Reserved
        100000000-13fffffff : System RAM
        140000000-33fffffff : virtio0
          140000000-1bfffffff : System RAM (virtio_mem)
        3280000000-32ffffffff : PCI Bus 0000:00

Of course, with more hotplugged memory, it gets worse. When unplugging
memory blocks again, try_remove_memory() (via
offline_and_remove_memory()) will properly split the resource up again.

Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Michal Hocko <mhocko@...e.com>
Cc: Dan Williams <dan.j.williams@...el.com>
Cc: Michael S. Tsirkin <mst@...hat.com>
Cc: Jason Wang <jasowang@...hat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@...il.com>
Cc: Baoquan He <bhe@...hat.com>
Cc: Wei Yang <richardw.yang@...ux.intel.com>
Signed-off-by: David Hildenbrand <david@...hat.com>
---
 drivers/virtio/virtio_mem.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio_mem.c b/drivers/virtio/virtio_mem.c
index 314ab753139d1..ba4de598f6636 100644
--- a/drivers/virtio/virtio_mem.c
+++ b/drivers/virtio/virtio_mem.c
@@ -424,7 +424,8 @@ static int virtio_mem_mb_add(struct virtio_mem *vm, unsigned long mb_id)
 
 	dev_dbg(&vm->vdev->dev, "adding memory block: %lu\n", mb_id);
 	return add_memory_driver_managed(nid, addr, memory_block_size_bytes(),
-					 vm->resource_name, 0);
+					 vm->resource_name,
+					 MEMHP_MERGE_RESOURCE);
 }
 
 /*
-- 
2.26.2