linux-kernel - Re: [PATCH 3/3] dax/kmem: Always enroll hotplugged memory for memmap_on

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Date:   Fri, 16 Jun 2023 09:54:22 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Vishal Verma <vishal.l.verma@...el.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Jiang <dave.jiang@...el.com>
Cc:     linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, nvdimm@...ts.linux.dev,
        linux-cxl@...r.kernel.org, Huang Ying <ying.huang@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 3/3] dax/kmem: Always enroll hotplugged memory for
 memmap_on_memory

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@...nel.org>
> Cc: Len Brown <lenb@...nel.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Dave Jiang <dave.jiang@...el.com>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Huang Ying <ying.huang@...el.com>
> Signed-off-by: Vishal Verma <vishal.l.verma@...el.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 ...

-- 
Cheers,

David / dhildenb