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

["Huang, Ying" <ying.huang@intel.com>]

Vishal Verma <vishal.l.verma@intel.com> writes:

> 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++;
>  	}

It appears that we need to hot-remove memory in the granularity of
memory_block_size_bytes() too, according to try_remove_memory().  If so,
it seems better to allocate one dax_kmem_data.res[] element for each
memory block instead of dax region?

Best Regards,
Huang, Ying