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Date: Thu, 2 May 2024 09:17:05 +0200
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Subject: Re: [PATCH v2 2/2] hv_balloon: Enable hot-add for memblock sizes >
 128 MiB
To: mhklinux@outlook.com, haiyangz@microsoft.com, wei.liu@kernel.org,
 decui@microsoft.com, linux-kernel@vger.kernel.org,
 linux-hyperv@vger.kernel.org
References: <20240501151458.2807-1-mhklinux@outlook.com>
 <20240501151458.2807-2-mhklinux@outlook.com>
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From: David Hildenbrand <david@redhat.com>
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In-Reply-To: <20240501151458.2807-2-mhklinux@outlook.com>
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On 01.05.24 17:14, mhkelley58@gmail.com wrote:
> From: Michael Kelley <mhklinux@outlook.com>
> 
> The Hyper-V balloon driver supports hot-add of memory in addition
> to ballooning. Current code hot-adds in fixed size chunks of
> 128 MiB (fixed constant HA_CHUNK in the code). While this works
> in Hyper-V VMs with 64 GiB or less or memory where the Linux
> memblock size is 128 MiB, the hot-add fails for larger memblock
> sizes because add_memory() expects memory to be added in chunks
> that match the memblock size. Messages like the following are
> reported when Linux has a 256 MiB memblock size:
> 
> [  312.668859] Block size [0x10000000] unaligned hotplug range:
>                 start 0x310000000, size 0x8000000
> [  312.668880] hv_balloon: hot_add memory failed error is -22
> [  312.668984] hv_balloon: Memory hot add failed
> 
> Larger memblock sizes are usually used in VMs with more than
> 64 GiB of memory, depending on the alignment of the VM's
> physical address space.
> 
> Fix this problem by having the Hyper-V balloon driver determine
> the Linux memblock size, and process hot-add requests in that
> chunk size instead of a fixed 128 MiB. Also update the hot-add
> alignment requested of the Hyper-V host to match the memblock
> size.
> 
> The code changes look significant, but in fact are just a
> simple text substitution of a new global variable for the
> previous HA_CHUNK constant. No algorithms are changed except
> to initialize the new global variable and to calculate the
> alignment value to pass to Hyper-V. Testing with memblock
> sizes of 256 MiB and 2 GiB shows correct operation.
> 
> Signed-off-by: Michael Kelley <mhklinux@outlook.com>
> ---
> Changes in v2:
> * Change new global variable name from ha_chunk_pgs to
>    ha_pages_in_chunk [David Hildenbrand]
> * Use kernel macros ALIGN(), ALIGN_DOWN(), and umin()
>    to simplify code and reduce references to HA_CHUNK. For
>    ease of review, this is done in a new patch preceeding
>    this one. [David Hildenbrand]
> 
>   drivers/hv/hv_balloon.c | 55 +++++++++++++++++++++++++----------------
>   1 file changed, 34 insertions(+), 21 deletions(-)
> 
> diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
> index 9f45b8a6762c..e0a1a18041ca 100644
> --- a/drivers/hv/hv_balloon.c
> +++ b/drivers/hv/hv_balloon.c
> @@ -425,11 +425,11 @@ struct dm_info_msg {
>    * The range start_pfn : end_pfn specifies the range
>    * that the host has asked us to hot add. The range
>    * start_pfn : ha_end_pfn specifies the range that we have
> - * currently hot added. We hot add in multiples of 128M
> - * chunks; it is possible that we may not be able to bring
> - * online all the pages in the region. The range
> + * currently hot added. We hot add in chunks equal to the
> + * memory block size; it is possible that we may not be able
> + * to bring online all the pages in the region. The range
>    * covered_start_pfn:covered_end_pfn defines the pages that can
> - * be brough online.
> + * be brought online.
>    */
>   
>   struct hv_hotadd_state {
> @@ -505,8 +505,9 @@ enum hv_dm_state {
>   
>   static __u8 recv_buffer[HV_HYP_PAGE_SIZE];
>   static __u8 balloon_up_send_buffer[HV_HYP_PAGE_SIZE];
> +static unsigned long ha_pages_in_chunk;
> +
>   #define PAGES_IN_2M (2 * 1024 * 1024 / PAGE_SIZE)
> -#define HA_CHUNK (128 * 1024 * 1024 / PAGE_SIZE)
>   
>   struct hv_dynmem_device {
>   	struct hv_device *dev;
> @@ -724,21 +725,21 @@ static void hv_mem_hot_add(unsigned long start, unsigned long size,
>   	unsigned long processed_pfn;
>   	unsigned long total_pfn = pfn_count;
>   
> -	for (i = 0; i < (size/HA_CHUNK); i++) {
> -		start_pfn = start + (i * HA_CHUNK);
> +	for (i = 0; i < (size/ha_pages_in_chunk); i++) {
> +		start_pfn = start + (i * ha_pages_in_chunk);
>   
>   		scoped_guard(spinlock_irqsave, &dm_device.ha_lock) {
> -			has->ha_end_pfn +=  HA_CHUNK;
> -			processed_pfn = umin(total_pfn, HA_CHUNK);
> +			has->ha_end_pfn += ha_pages_in_chunk;
> +			processed_pfn = umin(total_pfn, ha_pages_in_chunk);
>   			total_pfn -= processed_pfn;
> -			has->covered_end_pfn +=  processed_pfn;
> +			has->covered_end_pfn += processed_pfn;
>   		}
>   
>   		reinit_completion(&dm_device.ol_waitevent);
>   
>   		nid = memory_add_physaddr_to_nid(PFN_PHYS(start_pfn));
>   		ret = add_memory(nid, PFN_PHYS((start_pfn)),
> -				(HA_CHUNK << PAGE_SHIFT), MHP_MERGE_RESOURCE);
> +				(ha_pages_in_chunk << PAGE_SHIFT), MHP_MERGE_RESOURCE);
>   

HA_BYTES_IN_CHUNK might be reasonable to have (see below)

>   	if (do_hot_add)
> @@ -1807,10 +1808,13 @@ static int balloon_connect_vsp(struct hv_device *dev)
>   	cap_msg.caps.cap_bits.hot_add = hot_add_enabled();
>   
>   	/*
> -	 * Specify our alignment requirements as it relates
> -	 * memory hot-add. Specify 128MB alignment.
> +	 * Specify our alignment requirements for memory hot-add. The value is
> +	 * the log base 2 of the number of megabytes in a chunk. For example,
> +	 * with 256 MiB chunks, the value is 8. The number of MiB in a chunk
> +	 * must be a power of 2.
>   	 */
> -	cap_msg.caps.cap_bits.hot_add_alignment = 7;
> +	cap_msg.caps.cap_bits.hot_add_alignment =
> +			ilog2(ha_pages_in_chunk >> (20 - PAGE_SHIFT));

I was wondering if we can remove some of the magic here. Something along 
the lines of:

ilog2(ha_pages_in_chunk / (SZ_1M >> PAGE_SHIFT))

or simply

#define HA_BYTES_IN_CHUNK (ha_pages_in_chunk << PAGE_SHIFT)

ilog2(HA_BYTES_IN_CHUNK / SZ_1M)


Apart from that nothing jumped at me; looks much cleaner.

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb