[PATCH v3 0/2] mm: use memmap_on_memory semantics for dax/kmem

[PATCH v3 0/2] mm: use memmap_on_memory semantics for dax/kmem

[Vishal Verma]

The dax/kmem driver can potentially hot-add large amounts of memory
originating from CXL memory expanders, or NVDIMMs, or other 'device
memories'. There is a chance there isn't enough regular system memory
available to fit the memmap for this new memory. It's therefore
desirable, if all other conditions are met, for the kmem managed memory
to place its memmap on the newly added memory itself.

The main hurdle for accomplishing this for kmem is that memmap_on_memory
can only be done if the memory being added is equal to the size of one
memblock. To overcome this, allow the hotplug code to split an add_memory()
request into memblock-sized chunks, and try_remove_memory() to also
expect and handle such a scenario.

Patch 1 teaches the memory_hotplug code to allow for splitting
add_memory() and remove_memory() requests over memblock sized chunks.

Patch 2 adds a sysfs control for the kmem driver that would
allow an opt-out of using memmap_on_memory for the memory being added.

Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
---
Changes in v3:
- Rebase on Aneesh's patches [1]
- Drop Patch 1 - it is not needed since [1] allows for dynamic setting
  of the memmap_on_memory param (David)
- Link to v2: https://lore.kernel.org/r/20230720-vv-kmem_memmap-v2-0-88bdaab34993@intel.com

[1]: https://lore.kernel.org/r/20230801044116.10674-1-aneesh.kumar@linux.ibm.com

Changes in v2:
- Drop the patch to create an override path for the memmap_on_memory
  module param (David)
- Move the chunking into memory_hotplug.c so that any caller of
  add_memory() can request this behavior. (David)
- Handle remove_memory() too. (David, Ying)
- Add a sysfs control in the kmem driver for memmap_on_memory semantics
  (David, Jonathan)
- Add a #else case to define mhp_supports_memmap_on_memory() if
  CONFIG_MEMORY_HOTPLUG is unset. (0day report)
- Link to v1: https://lore.kernel.org/r/20230613-vv-kmem_memmap-v1-0-f6de9c6af2c6@intel.com

---
Vishal Verma (2):
      mm/memory_hotplug: split memmap_on_memory requests across memblocks
      dax/kmem: allow kmem to add memory with memmap_on_memory

 drivers/dax/dax-private.h |   1 +
 drivers/dax/bus.c         |  42 +++++++++++++
 drivers/dax/kmem.c        |   8 ++-
 mm/memory_hotplug.c       | 150 ++++++++++++++++++++++++++++------------------
 4 files changed, 143 insertions(+), 58 deletions(-)
---
base-commit: 64caf0ab00a1fc954cbf0aa43d65ca70d71d614f
change-id: 20230613-vv-kmem_memmap-5483c8d04279

Best regards,
-- 
Vishal Verma <vishal.l.verma@intel.com>

[PATCH v3 1/2] mm/memory_hotplug: split memmap_on_memory requests across memblocks

[Vishal Verma]

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.

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>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
---
 mm/memory_hotplug.c | 150 ++++++++++++++++++++++++++++++++--------------------
 1 file changed, 93 insertions(+), 57 deletions(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index d282664f558e..cae03c8d4bbf 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1383,6 +1383,44 @@ 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 = kmalloc(sizeof(struct vmem_altmap), GFP_KERNEL);
+		if (!params.altmap)
+			return -ENOMEM;
+
+		memcpy(params.altmap, &mhp_altmap, sizeof(mhp_altmap));
+	}
+
+	/* 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) {
+		arch_remove_memory(start, size, NULL);
+		goto error;
+	}
+
+	return 0;
+
+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).
@@ -1391,14 +1429,10 @@ 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;
 
@@ -1439,28 +1473,21 @@ 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 = kmalloc(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);
+			if (ret)
 				goto error;
-
-			memcpy(params.altmap, &mhp_altmap, sizeof(mhp_altmap));
 		}
-		/* 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) {
@@ -1497,8 +1524,6 @@ 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);
@@ -2149,40 +2174,14 @@ void try_offline_node(int nid)
 }
 EXPORT_SYMBOL(try_offline_node);
 
-static int __ref try_remove_memory(u64 start, u64 size)
+static void __ref __try_remove_memory(int nid, u64 start, u64 size)
 {
-	int ret;
-	struct memory_block *mem;
-	int rc = 0, nid = NUMA_NO_NODE;
 	struct vmem_altmap *altmap = NULL;
+	struct memory_block *mem;
+	int ret;
 
-	BUG_ON(check_hotplug_memory_range(start, size));
-
-	/*
-	 * All memory blocks must be offlined before removing memory.  Check
-	 * whether all memory blocks in question are offline and return error
-	 * if this is not the case.
-	 *
-	 * While at it, determine the nid. Note that if we'd have mixed nodes,
-	 * we'd only try to offline the last determined one -- which is good
-	 * enough for the cases we care about.
-	 */
-	rc = walk_memory_blocks(start, size, &nid, check_memblock_offlined_cb);
-	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.
-	 */
 	ret = walk_memory_blocks(start, size, &mem, test_has_altmap_cb);
 	if (ret) {
-		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
@@ -2221,6 +2220,43 @@ static int __ref try_remove_memory(u64 start, u64 size)
 		try_offline_node(nid);
 
 	mem_hotplug_done();
+}
+
+static int __ref try_remove_memory(u64 start, u64 size)
+{
+	int ret, nid = NUMA_NO_NODE;
+
+	BUG_ON(check_hotplug_memory_range(start, size));
+
+	/*
+	 * All memory blocks must be offlined before removing memory.  Check
+	 * whether all memory blocks in question are offline and return error
+	 * if this is not the case.
+	 *
+	 * While at it, determine the nid. Note that if we'd have mixed nodes,
+	 * we'd only try to offline the last determined one -- which is good
+	 * enough for the cases we care about.
+	 */
+	ret = walk_memory_blocks(start, size, &nid, check_memblock_offlined_cb);
+	if (ret)
+		return ret;
+
+	/*
+	 * For memmap_on_memory, the altmaps could have been added on
+	 * a per-memblock basis. Loop through the entire range if so,
+	 * and remove each memblock and its altmap.
+	 */
+	if (mhp_memmap_on_memory()) {
+		unsigned long memblock_size = memory_block_size_bytes();
+		u64 cur_start;
+
+		for (cur_start = start; cur_start < start + size;
+		     cur_start += memblock_size)
+			__try_remove_memory(nid, cur_start, memblock_size);
+	} else {
+		__try_remove_memory(nid, start, size);
+	}
+
 	return 0;
 }
 

-- 
2.41.0

Re: [PATCH v3 1/2] mm/memory_hotplug: split memmap_on_memory requests across memblocks

[Jonathan Cameron]

On Tue, 01 Aug 2023 23:55:37 -0600
Vishal Verma <vishal.l.verma@intel.com> 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.
> 
> 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>
> Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>

A couple of trivial comments inline.

> ---
>  mm/memory_hotplug.c | 150 ++++++++++++++++++++++++++++++++--------------------
>  1 file changed, 93 insertions(+), 57 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index d282664f558e..cae03c8d4bbf 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1383,6 +1383,44 @@ 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 = kmalloc(sizeof(struct vmem_altmap), GFP_KERNEL);
> +		if (!params.altmap)
> +			return -ENOMEM;
> +
> +		memcpy(params.altmap, &mhp_altmap, sizeof(mhp_altmap));
> +	}
> +
> +	/* 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) {
> +		arch_remove_memory(start, size, NULL);

Maybe push this down to a second label?

> +		goto error;
> +	}
> +
> +	return 0;
> +
> +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).
> @@ -1391,14 +1429,10 @@ 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;
>  
> @@ -1439,28 +1473,21 @@ 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 = kmalloc(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);
> +			if (ret)
>  				goto error;
> -
> -			memcpy(params.altmap, &mhp_altmap, sizeof(mhp_altmap));
>  		}
> -		/* 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) {
> @@ -1497,8 +1524,6 @@ 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);
> @@ -2149,40 +2174,14 @@ void try_offline_node(int nid)
>  }
>  EXPORT_SYMBOL(try_offline_node);
>  
> -static int __ref try_remove_memory(u64 start, u64 size)
> +static void __ref __try_remove_memory(int nid, u64 start, u64 size)
>  {
> -	int ret;
> -	struct memory_block *mem;
> -	int rc = 0, nid = NUMA_NO_NODE;
>  	struct vmem_altmap *altmap = NULL;
> +	struct memory_block *mem;
> +	int ret;
>  
> -	BUG_ON(check_hotplug_memory_range(start, size));
> -
> -	/*
> -	 * All memory blocks must be offlined before removing memory.  Check
> -	 * whether all memory blocks in question are offline and return error
> -	 * if this is not the case.
> -	 *
> -	 * While at it, determine the nid. Note that if we'd have mixed nodes,
> -	 * we'd only try to offline the last determined one -- which is good
> -	 * enough for the cases we care about.
> -	 */
> -	rc = walk_memory_blocks(start, size, &nid, check_memblock_offlined_cb);
> -	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.
> -	 */
>  	ret = walk_memory_blocks(start, size, &mem, test_has_altmap_cb);
>  	if (ret) {
> -		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
> @@ -2221,6 +2220,43 @@ static int __ref try_remove_memory(u64 start, u64 size)
>  		try_offline_node(nid);
>  
>  	mem_hotplug_done();
> +}
> +
> +static int __ref try_remove_memory(u64 start, u64 size)
> +{
> +	int ret, nid = NUMA_NO_NODE;

I'm not overly keen to see the trivial rename of rc -> ret in here.
Just makes it ever so slightly harder to compare old code and new code.

> +
> +	BUG_ON(check_hotplug_memory_range(start, size));
> +
> +	/*
> +	 * All memory blocks must be offlined before removing memory.  Check
> +	 * whether all memory blocks in question are offline and return error
> +	 * if this is not the case.
> +	 *
> +	 * While at it, determine the nid. Note that if we'd have mixed nodes,
> +	 * we'd only try to offline the last determined one -- which is good
> +	 * enough for the cases we care about.
> +	 */
> +	ret = walk_memory_blocks(start, size, &nid, check_memblock_offlined_cb);
> +	if (ret)
> +		return ret;
> +
> +	/*
> +	 * For memmap_on_memory, the altmaps could have been added on
> +	 * a per-memblock basis. Loop through the entire range if so,
> +	 * and remove each memblock and its altmap.
> +	 */
> +	if (mhp_memmap_on_memory()) {
> +		unsigned long memblock_size = memory_block_size_bytes();
> +		u64 cur_start;
> +
> +		for (cur_start = start; cur_start < start + size;
> +		     cur_start += memblock_size)
> +			__try_remove_memory(nid, cur_start, memblock_size);
> +	} else {
> +		__try_remove_memory(nid, start, size);
> +	}
> +
>  	return 0;
>  }
>  
> 

Re: [PATCH v3 1/2] mm/memory_hotplug: split memmap_on_memory requests across memblocks

[Verma, Vishal L]

On Wed, 2023-08-02 at 15:20 +0100, Jonathan Cameron wrote:
> On Tue, 01 Aug 2023 23:55:37 -0600
> Vishal Verma <vishal.l.verma@intel.com> 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.
> > 
> > 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>
> > Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
> 
> A couple of trivial comments inline.

Hi Jonathan,

Thanks for taking a look.

> 
> > ---
> >  mm/memory_hotplug.c | 150 ++++++++++++++++++++++++++++++++--------------------
> >  1 file changed, 93 insertions(+), 57 deletions(-)
> > 
> > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> > index d282664f558e..cae03c8d4bbf 100644
> > --- a/mm/memory_hotplug.c
> > +++ b/mm/memory_hotplug.c
> > @@ -1383,6 +1383,44 @@ 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 = kmalloc(sizeof(struct vmem_altmap), GFP_KERNEL);
> > +               if (!params.altmap)
> > +                       return -ENOMEM;
> > +
> > +               memcpy(params.altmap, &mhp_altmap, sizeof(mhp_altmap));
> > +       }
> > +
> > +       /* 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) {
> > +               arch_remove_memory(start, size, NULL);
> 
> Maybe push this down to a second label?

Yep will do.

> 
<snip>
> > +
> > +static int __ref try_remove_memory(u64 start, u64 size)
> > +{
> > +       int ret, nid = NUMA_NO_NODE;
> 
> I'm not overly keen to see the trivial rename of rc -> ret in here.
> Just makes it ever so slightly harder to compare old code and new code.

Yep - this was to work around the patches I was based on, which added
both a ret and left the original rc [1]. Aneesh will stick to 'rc' so
my next revision should sort this out naturally.

[1]: https://lore.kernel.org/all/715042319ceb86016a4986862a82756e5629d725.camel@intel.com/

> 

[PATCH v3 2/2] dax/kmem: allow kmem to add memory with memmap_on_memory

[Vishal Verma]

Large amounts of memory managed by the kmem driver may come in via CXL,
and it is often desirable to have the memmap for this memory on the new
memory itself.

Enroll kmem-managed memory for memmap_on_memory semantics as a default.
Add a sysfs override under the dax device to opt out of this behavior.

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>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
---
 drivers/dax/dax-private.h |  1 +
 drivers/dax/bus.c         | 42 ++++++++++++++++++++++++++++++++++++++++++
 drivers/dax/kmem.c        |  8 +++++++-
 3 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/drivers/dax/dax-private.h b/drivers/dax/dax-private.h
index 27cf2daaaa79..446617b73aea 100644
--- a/drivers/dax/dax-private.h
+++ b/drivers/dax/dax-private.h
@@ -70,6 +70,7 @@ struct dev_dax {
 	struct ida ida;
 	struct device dev;
 	struct dev_pagemap *pgmap;
+	bool memmap_on_memory;
 	int nr_range;
 	struct dev_dax_range {
 		unsigned long pgoff;
diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
index 0ee96e6fc426..e9c0349b6244 100644
--- a/drivers/dax/bus.c
+++ b/drivers/dax/bus.c
@@ -1269,6 +1269,40 @@ static ssize_t numa_node_show(struct device *dev,
 }
 static DEVICE_ATTR_RO(numa_node);
 
+static ssize_t memmap_on_memory_show(struct device *dev,
+				     struct device_attribute *attr, char *buf)
+{
+	struct dev_dax *dev_dax = to_dev_dax(dev);
+
+	return sprintf(buf, "%d\n", dev_dax->memmap_on_memory);
+}
+
+static ssize_t memmap_on_memory_store(struct device *dev,
+				      struct device_attribute *attr,
+				      const char *buf, size_t len)
+{
+	struct dev_dax *dev_dax = to_dev_dax(dev);
+	struct dax_region *dax_region = dev_dax->region;
+	ssize_t rc;
+	bool val;
+
+	rc = kstrtobool(buf, &val);
+	if (rc)
+		return rc;
+
+	device_lock(dax_region->dev);
+	if (!dax_region->dev->driver) {
+		device_unlock(dax_region->dev);
+		return -ENXIO;
+	}
+
+	dev_dax->memmap_on_memory = val;
+
+	device_unlock(dax_region->dev);
+	return rc == 0 ? len : rc;
+}
+static DEVICE_ATTR_RW(memmap_on_memory);
+
 static umode_t dev_dax_visible(struct kobject *kobj, struct attribute *a, int n)
 {
 	struct device *dev = container_of(kobj, struct device, kobj);
@@ -1295,6 +1329,7 @@ static struct attribute *dev_dax_attributes[] = {
 	&dev_attr_align.attr,
 	&dev_attr_resource.attr,
 	&dev_attr_numa_node.attr,
+	&dev_attr_memmap_on_memory.attr,
 	NULL,
 };
 
@@ -1400,6 +1435,13 @@ struct dev_dax *devm_create_dev_dax(struct dev_dax_data *data)
 	dev_dax->align = dax_region->align;
 	ida_init(&dev_dax->ida);
 
+	/*
+	 * If supported by memory_hotplug, allow memmap_on_memory behavior by
+	 * default. This can be overridden via sysfs before handing the memory
+	 * over to kmem if desired.
+	 */
+	dev_dax->memmap_on_memory = true;
+
 	inode = dax_inode(dax_dev);
 	dev->devt = inode->i_rdev;
 	dev->bus = &dax_bus_type;
diff --git a/drivers/dax/kmem.c b/drivers/dax/kmem.c
index c57acb73e3db..0aa6c45a4e5a 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"
 
@@ -56,6 +57,7 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
 	unsigned long total_len = 0;
 	struct dax_kmem_data *data;
 	int i, rc, mapped = 0;
+	mhp_t mhp_flags;
 	int numa_node;
 
 	/*
@@ -136,12 +138,16 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
 		 */
 		res->flags = IORESOURCE_SYSTEM_RAM;
 
+		mhp_flags = MHP_NID_IS_MGID;
+		if (dev_dax->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, range.start,
-				range_len(&range), kmem_name, MHP_NID_IS_MGID);
+				range_len(&range), kmem_name, mhp_flags);
 
 		if (rc) {
 			dev_warn(dev, "mapping%d: %#llx-%#llx memory add failed\n",

-- 
2.41.0

Re: [PATCH v3 2/2] dax/kmem: allow kmem to add memory with memmap_on_memory

[Jonathan Cameron]

On Tue, 01 Aug 2023 23:55:38 -0600
Vishal Verma <vishal.l.verma@intel.com> wrote:

> Large amounts of memory managed by the kmem driver may come in via CXL,
> and it is often desirable to have the memmap for this memory on the new
> memory itself.
> 
> Enroll kmem-managed memory for memmap_on_memory semantics as a default.
> Add a sysfs override under the dax device to opt out of this behavior.
> 
> 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>
> Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>

Hi Vishal,

In general looks fine to me.  Just a question for potential discussion if didn't
miss it in earlier versions.

FWIW
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Also, any docs need updating?  Doesn't seem like the DAX ABI docs are present in
Documentation/ABI so not sure where it should be updated.

Jonathan


>  
> @@ -1400,6 +1435,13 @@ struct dev_dax *devm_create_dev_dax(struct dev_dax_data *data)
>  	dev_dax->align = dax_region->align;
>  	ida_init(&dev_dax->ida);
>  
> +	/*
> +	 * If supported by memory_hotplug, allow memmap_on_memory behavior by
> +	 * default. This can be overridden via sysfs before handing the memory
> +	 * over to kmem if desired.
> +	 */
> +	dev_dax->memmap_on_memory = true;

If there are existing users, then this is a fairly significant change of defaults.
Maybe it should be false and opt in rather than out?

> +
>  	inode = dax_inode(dax_dev);
>  	dev->devt = inode->i_rdev;
>  	dev->bus = &dax_bus_type;