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Message-ID: <f2720382-5374-4b8f-ad3c-f3f13e4a3767@redhat.com>
Date:   Tue, 17 Oct 2023 16:49:38 +0200
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User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH v6 2/3] mm/memory_hotplug: split memmap_on_memory requests
 across memblocks
Content-Language: en-US
To:     Vishal Verma <vishal.l.verma@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Jiang <dave.jiang@intel.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        nvdimm@lists.linux.dev, linux-cxl@vger.kernel.org,
        Huang Ying <ying.huang@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        Michal Hocko <mhocko@suse.com>,
        Jonathan Cameron <Jonathan.Cameron@Huawei.com>,
        Jeff Moyer <jmoyer@redhat.com>
References: <20231016-vv-kmem_memmap-v6-0-078f0d3c0371@intel.com>
 <20231016-vv-kmem_memmap-v6-2-078f0d3c0371@intel.com>
From:   David Hildenbrand <david@redhat.com>
Autocrypt: addr=david@redhat.com; keydata=
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Organization: Red Hat
In-Reply-To: <20231016-vv-kmem_memmap-v6-2-078f0d3c0371@intel.com>
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Precedence: bulk

On 17.10.23 07:44, Vishal Verma wrote:
> The MHP_MEMMAP_ON_MEMORY flag for hotplugged memory is restricted to
> 'memblock_size' chunks of memory being added. Adding a larger span of
> memory precludes memmap_on_memory semantics.
> 
> For users of hotplug such as kmem, large amounts of memory might get
> added from the CXL subsystem. In some cases, this amount may exceed the
> available 'main memory' to store the memmap for the memory being added.
> In this case, it is useful to have a way to place the memmap on the
> memory being added, even if it means splitting the addition into
> memblock-sized chunks.
> 
> Change add_memory_resource() to loop over memblock-sized chunks of
> memory if caller requested memmap_on_memory, and if other conditions for
> it are met. Teach try_remove_memory() to also expect that a memory
> range being removed might have been split up into memblock sized chunks,
> and to loop through those as needed.
> 
> This does preclude being able to use PUD mappings in the direct map; a
> proposal to how this could be optimized in the future is laid out
> here[1].
> 
> [1]: https://lore.kernel.org/linux-mm/b6753402-2de9-25b2-36e9-eacd49752b19@redhat.com/
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Michal Hocko <mhocko@suse.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>
> Suggested-by: David Hildenbrand <david@redhat.com>
> Reviewed-by: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
> ---
>   mm/memory_hotplug.c | 214 ++++++++++++++++++++++++++++++++++++----------------
>   1 file changed, 148 insertions(+), 66 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 6be7de9efa55..83e5ec377aad 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1380,6 +1380,43 @@ static bool mhp_supports_memmap_on_memory(unsigned long size)
>   	return arch_supports_memmap_on_memory(vmemmap_size);
>   }
>   
> +static int add_memory_create_devices(int nid, struct memory_group *group,
> +				     u64 start, u64 size, mhp_t mhp_flags)
> +{
> +	struct mhp_params params = { .pgprot = pgprot_mhp(PAGE_KERNEL) };
> +	struct vmem_altmap mhp_altmap = {
> +		.base_pfn =  PHYS_PFN(start),
> +		.end_pfn  =  PHYS_PFN(start + size - 1),
> +	};
> +	int ret;
> +
> +	if ((mhp_flags & MHP_MEMMAP_ON_MEMORY)) {
> +		mhp_altmap.free = memory_block_memmap_on_memory_pages();
> +		params.altmap = kmemdup(&mhp_altmap, sizeof(struct vmem_altmap),
> +					GFP_KERNEL);
> +		if (!params.altmap)
> +			return -ENOMEM;
> +	}
> +
> +	/* call arch's memory hotadd */
> +	ret = arch_add_memory(nid, start, size, &params);
> +	if (ret < 0)
> +		goto error;
> +
> +	/* create memory block devices after memory was added */
> +	ret = create_memory_block_devices(start, size, params.altmap, group);
> +	if (ret)
> +		goto err_bdev;
> +
> +	return 0;
> +
> +err_bdev:
> +	arch_remove_memory(start, size, NULL);
> +error:
> +	kfree(params.altmap);
> +	return ret;
> +}
> +
>   /*
>    * NOTE: The caller must call lock_device_hotplug() to serialize hotplug
>    * and online/offline operations (triggered e.g. by sysfs).
> @@ -1388,14 +1425,10 @@ static bool mhp_supports_memmap_on_memory(unsigned long size)
>    */
>   int __ref add_memory_resource(int nid, struct resource *res, mhp_t mhp_flags)
>   {
> -	struct mhp_params params = { .pgprot = pgprot_mhp(PAGE_KERNEL) };
> +	unsigned long memblock_size = memory_block_size_bytes();
>   	enum memblock_flags memblock_flags = MEMBLOCK_NONE;
> -	struct vmem_altmap mhp_altmap = {
> -		.base_pfn =  PHYS_PFN(res->start),
> -		.end_pfn  =  PHYS_PFN(res->end),
> -	};
>   	struct memory_group *group = NULL;
> -	u64 start, size;
> +	u64 start, size, cur_start;
>   	bool new_node = false;
>   	int ret;
>   
> @@ -1436,28 +1469,21 @@ int __ref add_memory_resource(int nid, struct resource *res, mhp_t mhp_flags)
>   	/*
>   	 * Self hosted memmap array
>   	 */
> -	if (mhp_flags & MHP_MEMMAP_ON_MEMORY) {
> -		if (mhp_supports_memmap_on_memory(size)) {
> -			mhp_altmap.free = memory_block_memmap_on_memory_pages();
> -			params.altmap = kmemdup(&mhp_altmap,
> -						sizeof(struct vmem_altmap),
> -						GFP_KERNEL);
> -			if (!params.altmap)
> +	if ((mhp_flags & MHP_MEMMAP_ON_MEMORY) &&
> +	    mhp_supports_memmap_on_memory(memblock_size)) {
> +		for (cur_start = start; cur_start < start + size;
> +		     cur_start += memblock_size) {
> +			ret = add_memory_create_devices(nid, group, cur_start,
> +							memblock_size,
> +							mhp_flags);

Just like on the removal part, that function doesn't make any sense to 
be called with altmap and a bigger range.

Maybe better also move that loop into a function called 
"create_altmap_and_memory_blocks()" , to be the opposite of 
"remove_memory_blocks_and_altmaps()".

Then you can keep the existing !altmap code here in the "else" case. 
Again, similar to the removal code below.

> +			if (ret)
>   				goto error;
>   		}
> -		/* fallback to not using altmap  */
> -	}
> -
> -	/* call arch's memory hotadd */
> -	ret = arch_add_memory(nid, start, size, &params);
> -	if (ret < 0)
> -		goto error_free;
> -
> -	/* create memory block devices after memory was added */
> -	ret = create_memory_block_devices(start, size, params.altmap, group);
> -	if (ret) {
> -		arch_remove_memory(start, size, NULL);
> -		goto error_free;
> +	} else {
> +		ret = add_memory_create_devices(nid, group, start, size,
> +						mhp_flags);
> +		if (ret)
> +			goto error;
>   	}
>   
>   	if (new_node) {
> @@ -1494,8 +1520,6 @@ int __ref add_memory_resource(int nid, struct resource *res, mhp_t mhp_flags)
>   		walk_memory_blocks(start, size, NULL, online_memory_block);
>   
>   	return ret;
> -error_free:
> -	kfree(params.altmap);
>   error:
>   	if (IS_ENABLED(CONFIG_ARCH_KEEP_MEMBLOCK))
>   		memblock_remove(start, size);
> @@ -2076,6 +2100,23 @@ static int test_has_altmap_cb(struct memory_block *mem, void *arg)
>   	return 0;
>   }


[...]

> +
> +static int memblocks_have_altmaps(u64 start, u64 size)
> +{
> +	struct memory_range_altmap_info altmap_info = {
> +		.num_memblocks = 0,
> +		.num_altmaps = 0
> +	};

You can get the number of memblocks simply by "size / 
memory_block_size_bytes()". So you can avoid the "struct" and just pass 
an int.

> +
> +	if (!mhp_memmap_on_memory())
> +		return 0;
> +
> +	walk_memory_blocks(start, size, &altmap_info,
> +			   count_memory_range_altmaps_cb);
> +
> +	if (!altmap_info.num_altmaps)
> +		return 0;
> +
> +	if (altmap_info.num_memblocks != altmap_info.num_altmaps) {
> +		WARN_ONCE(1, "Not all memblocks in range have altmaps");
> +		return -EINVAL;
> +	}
> +
> +	return 1;
> +}
> +
> +static int __ref try_remove_memory(u64 start, u64 size)
> +{
> +	int rc, nid = NUMA_NO_NODE;
>   
>   	BUG_ON(check_hotplug_memory_range(start, size));
>   
> @@ -2167,45 +2272,21 @@ static int __ref try_remove_memory(u64 start, u64 size)
>   	if (rc)
>   		return rc;
>   
> -	/*
> -	 * We only support removing memory added with MHP_MEMMAP_ON_MEMORY in
> -	 * the same granularity it was added - a single memory block.
> -	 */
> -	if (mhp_memmap_on_memory()) {
> -		rc = walk_memory_blocks(start, size, &mem, test_has_altmap_cb);
> -		if (rc) {
> -			if (size != memory_block_size_bytes()) {
> -				pr_warn("Refuse to remove %#llx - %#llx,"
> -					"wrong granularity\n",
> -					start, start + size);
> -				return -EINVAL;
> -			}
> -			altmap = mem->altmap;
> -			/*
> -			 * Mark altmap NULL so that we can add a debug
> -			 * check on memblock free.
> -			 */
> -			mem->altmap = NULL;
> -		}
> -	}
> -
>   	/* remove memmap entry */
>   	firmware_map_remove(start, start + size, "System RAM");
>   
> -	/*
> -	 * Memory block device removal under the device_hotplug_lock is
> -	 * a barrier against racing online attempts.
> -	 */

Maybe move that comment into the else branch and not into 
remove_memory_blocks_and_altmaps

> -	remove_memory_block_devices(start, size);
> -
>   	mem_hotplug_begin();
>   
> -	arch_remove_memory(start, size, altmap);
> -
> -	/* Verify that all vmemmap pages have actually been freed. */
> -	if (altmap) {
> -		WARN(altmap->alloc, "Altmap not fully unmapped");
> -		kfree(altmap);
> +	rc = memblocks_have_altmaps(start, size);

Don't use the term memblocks here, to not confuse it with the memblock 
allocator.

"memory blocks" or "memory block devices". it's complicated.

> +	if (rc < 0) {
> +		goto err;
> +	} else if (rc == 0) {
> +		/* no altmaps, do the removal directly */
> +		remove_memory_block_devices(start, size);
> +		arch_remove_memory(start, size, NULL);
> +	} else {
> +		/* all memblocks in the range have altmaps */
> +		remove_memory_blocks_and_altmaps(start, size);
>   	}
>   
>   	if (IS_ENABLED(CONFIG_ARCH_KEEP_MEMBLOCK)) {
> @@ -2218,8 +2299,9 @@ static int __ref try_remove_memory(u64 start, u64 size)
>   	if (nid != NUMA_NO_NODE)
>   		try_offline_node(nid);
>   
> +err:
>   	mem_hotplug_done();
> -	return 0;
> +	return (rc < 0 ? rc : 0);
>   }
>   
>   /**
> 

-- 
Cheers,

David / dhildenb