Re: [PATCH v4 6/8] virtio-mem: try to merge system ram resources

[Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx>]

On Fri, Sep 11, 2020 at 12:34:57PM +0200, David Hildenbrand wrote:
>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.
>
>Reviewed-by: Pankaj Gupta <pankaj.gupta.linux@xxxxxxxxx>
>Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>Cc: Michal Hocko <mhocko@xxxxxxxx>
>Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
>Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
>Cc: Jason Wang <jasowang@xxxxxxxxxx>
>Cc: Pankaj Gupta <pankaj.gupta.linux@xxxxxxxxx>
>Cc: Baoquan He <bhe@xxxxxxxxxx>
>Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Reviewed-by: Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx>

>---
> 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 ed99e43354010..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, MHP_NONE);
>+                                       vm->resource_name,
>+                                       MEMHP_MERGE_RESOURCE);
> }
> 
> /*
>-- 
>2.26.2

-- 
Wei Yang
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