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Message-ID: <20260105203611.4079743-1-gourry@gourry.net>
Date: Mon, 5 Jan 2026 15:36:11 -0500
From: Gregory Price <gourry@...rry.net>
To: linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org,
kernel-team@...a.com,
david@...hat.com,
osalvador@...e.de,
gregkh@...uxfoundation.org,
rafael@...nel.org,
dakr@...nel.org,
akpm@...ux-foundation.org,
lorenzo.stoakes@...cle.com,
Liam.Howlett@...cle.com,
vbabka@...e.cz,
rppt@...nel.org,
surenb@...gle.com,
mhocko@...e.com,
hare@...e.de
Subject: [RFC PATCH] memory,memory_hotplug: allow restricting memory blocks to zone movable
It was reported (LPC 2025) that userland services which monitor memory
blocks can cause hot-unplug to fail permanently.
This can occur when drivers attempt to hot-remove memory in two phases
(offline, remove), while a userland service detects the memory offline
and re-onlines the memory into a zone which may prevent removal.
This patch allows a driver to specify that a given memory block is
intended as ZONE_MOVABLE memory only (i.e. the system should try to
protect its hot-unpluggability). This is done via an MHP flag and a new
"movable_only" bool in `struct memory_block`.
Attempts to online a memory block with movable_only=true with any value
other than MMOP_ONLINE_MOVABLE will fail with -EINVAL.
It is hard to catch all possible ways to implement offline/remove
process, so a race condition here can clearly still occur if the
userland service onlines the memory back into ZONE_MOVABLE, but it at
least will not prevent the removal of a block at a later time.
Suggested-by: Hannes Reinecke <hare@...e.de>
Signed-off-by: Gregory Price <gourry@...rry.net>
---
drivers/base/memory.c | 15 +++++++++++----
include/linux/memory.h | 4 +++-
include/linux/memory_hotplug.h | 13 +++++++++++++
mm/memory_hotplug.c | 12 +++++++++---
4 files changed, 36 insertions(+), 8 deletions(-)
diff --git a/drivers/base/memory.c b/drivers/base/memory.c
index 6d84a02cfa5d..59512e4b8d62 100644
--- a/drivers/base/memory.c
+++ b/drivers/base/memory.c
@@ -374,6 +374,8 @@ static int memory_block_change_state(struct memory_block *mem,
if (to_state == MEM_OFFLINE)
mem->state = MEM_GOING_OFFLINE;
+ else if (mem->movable_only && to_state != MMOP_ONLINE_MOVABLE)
+ return -EINVAL;
ret = memory_block_action(mem, to_state);
mem->state = ret ? from_state_req : to_state;
@@ -811,7 +813,8 @@ void memory_block_add_nid_early(struct memory_block *mem, int nid)
static int add_memory_block(unsigned long block_id, int nid, unsigned long state,
struct vmem_altmap *altmap,
- struct memory_group *group)
+ struct memory_group *group,
+ bool movable_only)
{
struct memory_block *mem;
int ret = 0;
@@ -829,6 +832,7 @@ static int add_memory_block(unsigned long block_id, int nid, unsigned long state
mem->state = state;
mem->nid = nid;
mem->altmap = altmap;
+ mem->movable_only = movable_only;
INIT_LIST_HEAD(&mem->group_next);
#ifndef CONFIG_NUMA
@@ -880,7 +884,8 @@ static void remove_memory_block(struct memory_block *memory)
*/
int create_memory_block_devices(unsigned long start, unsigned long size,
int nid, struct vmem_altmap *altmap,
- struct memory_group *group)
+ struct memory_group *group,
+ bool movable_only)
{
const unsigned long start_block_id = pfn_to_block_id(PFN_DOWN(start));
unsigned long end_block_id = pfn_to_block_id(PFN_DOWN(start + size));
@@ -893,7 +898,8 @@ int create_memory_block_devices(unsigned long start, unsigned long size,
return -EINVAL;
for (block_id = start_block_id; block_id != end_block_id; block_id++) {
- ret = add_memory_block(block_id, nid, MEM_OFFLINE, altmap, group);
+ ret = add_memory_block(block_id, nid, MEM_OFFLINE, altmap, group,
+ movable_only);
if (ret)
break;
}
@@ -998,7 +1004,8 @@ void __init memory_dev_init(void)
continue;
block_id = memory_block_id(nr);
- ret = add_memory_block(block_id, NUMA_NO_NODE, MEM_ONLINE, NULL, NULL);
+ ret = add_memory_block(block_id, NUMA_NO_NODE, MEM_ONLINE, NULL, NULL,
+ false);
if (ret) {
panic("%s() failed to add memory block: %d\n",
__func__, ret);
diff --git a/include/linux/memory.h b/include/linux/memory.h
index 43d378038ce2..bab24f796d3d 100644
--- a/include/linux/memory.h
+++ b/include/linux/memory.h
@@ -80,6 +80,7 @@ struct memory_block {
struct vmem_altmap *altmap;
struct memory_group *group; /* group (if any) for this block */
struct list_head group_next; /* next block inside memory group */
+ bool movable_only; /* If set, only ZONE_MOVABLE is valid */
#if defined(CONFIG_MEMORY_FAILURE) && defined(CONFIG_MEMORY_HOTPLUG)
atomic_long_t nr_hwpoison;
#endif
@@ -160,7 +161,8 @@ extern int register_memory_notifier(struct notifier_block *nb);
extern void unregister_memory_notifier(struct notifier_block *nb);
int create_memory_block_devices(unsigned long start, unsigned long size,
int nid, struct vmem_altmap *altmap,
- struct memory_group *group);
+ struct memory_group *group,
+ bool movable_only);
void remove_memory_block_devices(unsigned long start, unsigned long size);
extern void memory_dev_init(void);
extern int memory_notify(unsigned long val, void *v);
diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index 23f038a16231..ca51ef2ad0cf 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -75,6 +75,19 @@ typedef int __bitwise mhp_t;
*/
#define MHP_OFFLINE_INACCESSIBLE ((__force mhp_t)BIT(3))
+/*
+ * Restrict hotplugged memory blocks to ZONE_MOVABLE only.
+ *
+ * During offlining of hotplugged memory which was originally onlined
+ * as ZONE_MOVABLE, userland services may detect blocks going offline
+ * and automatically re-online them into ZONE_NORMAL or lower. When
+ * this happens it may become permanently incapable of being removed.
+ *
+ * Allow driver-managed memory sources to restrict memory blocks to
+ * ZONE_MOVABLE only, so that the truly degenerate case can be mitigated.
+ */
+#define MHP_MOVABLE_ONLY ((__force mhp_t)BIT(4))
+
/*
* Extended parameters for memory hotplug:
* altmap: alternative allocator for memmap array (optional)
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 81ba5b019926..1a184bfd87f6 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1346,7 +1346,9 @@ static int check_hotplug_memory_range(u64 start, u64 size)
static int online_memory_block(struct memory_block *mem, void *arg)
{
- mem->online_type = mhp_get_default_online_type();
+ mem->online_type = mem->movable_only ?
+ MMOP_ONLINE_MOVABLE :
+ mhp_get_default_online_type();
return device_online(&mem->dev);
}
@@ -1449,6 +1451,7 @@ static int create_altmaps_and_memory_blocks(int nid, struct memory_group *group,
unsigned long memblock_size = memory_block_size_bytes();
u64 cur_start;
int ret;
+ bool movable_only = mhp_flags & MHP_MOVABLE_ONLY;
for (cur_start = start; cur_start < start + size;
cur_start += memblock_size) {
@@ -1478,7 +1481,8 @@ static int create_altmaps_and_memory_blocks(int nid, struct memory_group *group,
/* create memory block devices after memory was added */
ret = create_memory_block_devices(cur_start, memblock_size, nid,
- params.altmap, group);
+ params.altmap, group,
+ movable_only);
if (ret) {
arch_remove_memory(cur_start, memblock_size, NULL);
kfree(params.altmap);
@@ -1506,6 +1510,7 @@ int add_memory_resource(int nid, struct resource *res, mhp_t mhp_flags)
struct memory_group *group = NULL;
u64 start, size;
bool new_node = false;
+ bool movable_only = mhp_flags & MHP_MOVABLE_ONLY;
int ret;
start = res->start;
@@ -1564,7 +1569,8 @@ int add_memory_resource(int nid, struct resource *res, mhp_t mhp_flags)
goto error;
/* create memory block devices after memory was added */
- ret = create_memory_block_devices(start, size, nid, NULL, group);
+ ret = create_memory_block_devices(start, size, nid, NULL, group,
+ movable_only);
if (ret) {
arch_remove_memory(start, size, params.altmap);
goto error;
--
2.52.0
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