Re: [PATCH 3/3] dax/kmem: Always enroll hotplugged memory for memmap_on_memory

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

David Hildenbrand <david@redhat.com> writes:

> On 16.06.23 00:00, Vishal Verma wrote:
>> With DAX memory regions originating from CXL memory expanders or
>> NVDIMMs, the kmem driver may be hot-adding huge amounts of system memory
>> on a system without enough 'regular' main memory to support the memmap
>> for it. To avoid this, ensure that all kmem managed hotplugged memory is
>> added with the MHP_MEMMAP_ON_MEMORY flag to place the memmap on the
>> new memory region being hot added.
>>
>> To do this, call add_memory() in chunks of memory_block_size_bytes() as
>> that is a requirement for memmap_on_memory. Additionally, Use the
>> mhp_flag to force the memmap_on_memory checks regardless of the
>> respective module parameter setting.
>>
>> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
>> Cc: Len Brown <lenb@kernel.org>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Oscar Salvador <osalvador@suse.de>
>> Cc: Dan Williams <dan.j.williams@intel.com>
>> Cc: Dave Jiang <dave.jiang@intel.com>
>> Cc: Dave Hansen <dave.hansen@linux.intel.com>
>> Cc: Huang Ying <ying.huang@intel.com>
>> Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
>> ---
>>   drivers/dax/kmem.c | 49 ++++++++++++++++++++++++++++++++++++-------------
>>   1 file changed, 36 insertions(+), 13 deletions(-)
>>
>> diff --git a/drivers/dax/kmem.c b/drivers/dax/kmem.c
>> index 7b36db6f1cbd..0751346193ef 100644
>> --- a/drivers/dax/kmem.c
>> +++ b/drivers/dax/kmem.c
>> @@ -12,6 +12,7 @@
>>   #include <linux/mm.h>
>>   #include <linux/mman.h>
>>   #include <linux/memory-tiers.h>
>> +#include <linux/memory_hotplug.h>
>>   #include "dax-private.h"
>>   #include "bus.h"
>>
>> @@ -105,6 +106,7 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
>>   	data->mgid = rc;
>>
>>   	for (i = 0; i < dev_dax->nr_range; i++) {
>> +		u64 cur_start, cur_len, remaining;
>>   		struct resource *res;
>>   		struct range range;
>>
>> @@ -137,21 +139,42 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
>>   		res->flags = IORESOURCE_SYSTEM_RAM;
>>
>>   		/*
>> -		 * Ensure that future kexec'd kernels will not treat
>> -		 * this as RAM automatically.
>> +		 * Add memory in chunks of memory_block_size_bytes() so that
>> +		 * it is considered for MHP_MEMMAP_ON_MEMORY
>> +		 * @range has already been aligned to memory_block_size_bytes(),
>> +		 * so the following loop will always break it down cleanly.
>>   		 */
>> -		rc = add_memory_driver_managed(data->mgid, range.start,
>> -				range_len(&range), kmem_name, MHP_NID_IS_MGID);
>> +		cur_start = range.start;
>> +		cur_len = memory_block_size_bytes();
>> +		remaining = range_len(&range);
>> +		while (remaining) {
>> +			mhp_t mhp_flags = MHP_NID_IS_MGID;
>>
>> -		if (rc) {
>> -			dev_warn(dev, "mapping%d: %#llx-%#llx memory add failed\n",
>> -					i, range.start, range.end);
>> -			remove_resource(res);
>> -			kfree(res);
>> -			data->res[i] = NULL;
>> -			if (mapped)
>> -				continue;
>> -			goto err_request_mem;
>> +			if (mhp_supports_memmap_on_memory(cur_len,
>> +							  MHP_MEMMAP_ON_MEMORY))
>> +				mhp_flags |= MHP_MEMMAP_ON_MEMORY;
>> +			/*
>> +			 * Ensure that future kexec'd kernels will not treat
>> +			 * this as RAM automatically.
>> +			 */
>> +			rc = add_memory_driver_managed(data->mgid, cur_start,
>> +						       cur_len, kmem_name,
>> +						       mhp_flags);
>> +
>> +			if (rc) {
>> +				dev_warn(dev,
>> +					 "mapping%d: %#llx-%#llx memory add failed\n",
>> +					 i, cur_start, cur_start + cur_len - 1);
>> +				remove_resource(res);
>> +				kfree(res);
>> +				data->res[i] = NULL;
>> +				if (mapped)
>> +					continue;
>> +				goto err_request_mem;
>> +			}
>> +
>> +			cur_start += cur_len;
>> +			remaining -= cur_len;
>>   		}
>>   		mapped++;
>>   	}
>>
>
> Maybe the better alternative is teach
> add_memory_resource()/try_remove_memory() to do that internally.
>
> In the add_memory_resource() case, it might be a loop around that
> memmap_on_memory + arch_add_memory code path (well, and the error path
> also needs adjustment):
>
> 	/*
> 	 * Self hosted memmap array
> 	 */
> 	if (mhp_flags & MHP_MEMMAP_ON_MEMORY) {
> 		if (!mhp_supports_memmap_on_memory(size)) {
> 			ret = -EINVAL;
> 			goto error;
> 		}
> 		mhp_altmap.free = PHYS_PFN(size);
> 		mhp_altmap.base_pfn = PHYS_PFN(start);
> 		params.altmap = &mhp_altmap;
> 	}
>
> 	/* call arch's memory hotadd */
> 	ret = arch_add_memory(nid, start, size, &params);
> 	if (ret < 0)
> 		goto error;
>
>
> Note that we want to handle that on a per memory-block basis, because we
> don't want the vmemmap of memory block #2 to end up on memory block #1.
> It all gets messy with memory onlining/offlining etc otherwise ...
>

I tried to implement this inside add_memory_driver_managed() and also
within dax/kmem. IMHO doing the error handling inside dax/kmem is
better. Here is how it looks:

1. If any blocks got added before (mapped > 0) we loop through all successful request_mem_regions
2. For each succesful request_mem_regions if any blocks got added, we
keep the resource. If none got added, we will kfree the resource



	for (i = 0; i < dev_dax->nr_range; i++) {
		u64 cur_start, cur_len, remaining;
		struct resource *res;
		struct range range;
		bool block_added;

		rc = dax_kmem_range(dev_dax, i, &range);
		if (rc)
			continue;

		/* Region is permanently reserved if hotremove fails. */
		res = request_mem_region(range.start, range_len(&range), data->res_name);
		if (!res) {
			dev_warn(dev, "mapping%d: %#llx-%#llx could not reserve region\n",
					i, range.start, range.end);
			/*
			 * Once some memory has been onlined we can't
			 * assume that it can be un-onlined safely.
			 */
			if (mapped)
				continue;
			rc = -EBUSY;
			goto err_request_mem;
		}
		data->res[i] = res;

		/*
		 * Set flags appropriate for System RAM.  Leave ..._BUSY clear
		 * so that add_memory() can add a child resource.  Do not
		 * inherit flags from the parent since it may set new flags
		 * unknown to us that will break add_memory() below.
		 */
		res->flags = IORESOURCE_SYSTEM_RAM;

		/*
		 * Add memory in chunks of memory_block_size_bytes() so that
		 * it is considered for MHP_MEMMAP_ON_MEMORY
		 * @range has already been aligned to memory_block_size_bytes(),
		 * so the following loop will always break it down cleanly.
		 */
		cur_start = range.start;
		cur_len = memory_block_size_bytes();
		remaining = range_len(&range);
		block_added = false;
		while (remaining) {
			/*
			 * If alignment rules are not satisified we will
			 * fallback normal memmap allocation.
			 */
			mhp_t mhp_flags = MHP_NID_IS_MGID | MHP_MEMMAP_ON_MEMORY;
			/*
			 * Ensure that future kexec'd kernels will not treat
			 * this as RAM automatically.
			 */
			rc = add_memory_driver_managed(data->mgid, cur_start,
						       cur_len, kmem_name,
						       mhp_flags);

			if (rc)
				dev_warn(dev,
					 "mapping%d: %#llx-%#llx memory add failed\n",
					 i, cur_start, cur_start + cur_len - 1);
			else
				block_added = true;

			cur_start += cur_len;
			remaining -= cur_len;
		}
		if (!block_added) {
			/*
			 * None of the blocks got added, remove the resource.
			 */
			remove_resource(res);
			kfree(res);
			data->res[i] = NULL;
		} else
			mapped++;
	}
	if (mapped) {
		dev_set_drvdata(dev, data);
		return 0;
	}

err_request_mem:
	/*
	 *  If none of the resources got mapped.
	 *  unregister the group.
	 */
	memory_group_unregister(data->mgid);
err_reg_mgid:
	kfree(data->res_name);