linux-kernel - Re: [PATCH RFCv1 3/5] virtio-mem: try to merge "System RAM (virtio

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Date:   Fri, 31 Jul 2020 12:40:19 +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,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH RFCv1 3/5] virtio-mem: try to merge "System RAM
 (virtio_mem)" resources

On 31.07.20 11:18, David Hildenbrand wrote:

Grml, forgot to add cc: list for this patch, ccing the right people.

> 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.
> 
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  drivers/virtio/virtio_mem.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_mem.c b/drivers/virtio/virtio_mem.c
> index f26f5f64ae822..2396a8d67875e 100644
> --- a/drivers/virtio/virtio_mem.c
> +++ b/drivers/virtio/virtio_mem.c
> @@ -415,6 +415,7 @@ static int virtio_mem_mb_add(struct virtio_mem *vm, unsigned long mb_id)
>  {
>  	const uint64_t addr = virtio_mem_mb_id_to_phys(mb_id);
>  	int nid = vm->nid;
> +	int rc;
>  
>  	if (nid == NUMA_NO_NODE)
>  		nid = memory_add_physaddr_to_nid(addr);
> @@ -431,8 +432,17 @@ 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);
> +	rc = add_memory_driver_managed(nid, addr, memory_block_size_bytes(),
> +				       vm->resource_name);
> +	if (!rc) {
> +		/*
> +		 * Try to reduce the number of resources by merging them. The
> +		 * memory removal path will properly split them up again.
> +		 */
> +		merge_child_mem_resources(vm->parent_resource,
> +					  vm->resource_name);
> +	}
> +	return rc;
>  }
>  
>  /*
> 


-- 
Thanks,

David / dhildenb