Re: [PATCH v5 1/2] mm/memory_hotplug: split memmap_on_memory requests across memblocks

From: David Hildenbrand
Date: Mon Oct 09 2023 - 11:04:56 EST

On 07.10.23 10:55, Huang, Ying wrote:
Vishal Verma <vishal.l.verma@xxxxxxxxx> writes:

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@xxxxxxxxxxxxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Dave Jiang <dave.jiang@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Huang Ying <ying.huang@xxxxxxxxx>
Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
Signed-off-by: Vishal Verma <vishal.l.verma@xxxxxxxxx>
---
mm/memory_hotplug.c | 162 ++++++++++++++++++++++++++++++++--------------------
1 file changed, 99 insertions(+), 63 deletions(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index f8d3e7427e32..77ec6f15f943 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1380,6 +1380,44 @@ 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)
+ 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 +1426,10 @@ 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 +1470,21 @@ 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) {
@@ -1494,8 +1521,6 @@ 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);
@@ -2146,12 +2171,41 @@ void try_offline_node(int nid)
}
EXPORT_SYMBOL(try_offline_node);
-static int __ref try_remove_memory(u64 start, u64 size)
+static void __ref remove_memory_block_and_altmap(int nid, u64 start, u64 size)
{
+ int rc = 0;
struct memory_block *mem;
- int rc = 0, nid = NUMA_NO_NODE;
struct vmem_altmap *altmap = NULL;
+ rc = walk_memory_blocks(start, size, &mem, test_has_altmap_cb);
+ if (rc) {
+ altmap = mem->altmap;
+ /*
+ * Mark altmap NULL so that we can add a debug
+ * check on memblock free.
+ */
+ mem->altmap = NULL;
+ }
+
+ /*
+ * Memory block device removal under the device_hotplug_lock is
+ * a barrier against racing online attempts.
+ */
+ remove_memory_block_devices(start, size);
+
+ 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);
+ }
+}
+
+static int __ref try_remove_memory(u64 start, u64 size)
+{
+ int rc, nid = NUMA_NO_NODE;
+
BUG_ON(check_hotplug_memory_range(start, size));
/*
@@ -2167,47 +2221,28 @@ static int __ref try_remove_memory(u64 start, u64 size)
if (rc)
return rc;
+ mem_hotplug_begin();
+
/*
- * We only support removing memory added with MHP_MEMMAP_ON_MEMORY in
- * the same granularity it was added - a single memory block.
+ * 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()) {

IIUC, even if mhp_memmap_on_memory() returns true, it's still possible
that the memmap is put in DRAM after [2/2]. So that,
arch_remove_memory() are called for each memory block unnecessarily. Can
we detect this (via altmap?) and call remove_memory_block_and_altmap()
for the whole range?

Good point. We should handle memblock-per-memblock onny if we have to handle the altmap. Otherwise, just call a separate function that doesn't care about -- e.g., called remove_memory_blocks_no_altmap().

We could simply walk all memory blocks and make sure either all have an altmap or none has an altmap. If there is a mix, we should bail out with WARN_ON_ONCE().

--
Cheers,

David / dhildenb