[PATCH v4 2/3] mm/page_owner: add NUMA node filter with nodelist support

[Zhen Ni <zhen.ni@easystack.cn>]

Add NUMA node filtering functionality to page_owner to allow filtering
pages by specific NUMA node(s). This is useful for NUMA-aware memory
allocation analysis and debugging.

The filter supports flexible nodelist input formats:
- Single node: echo "0" > nid
- Multiple nodes: echo "0,2,3" > nid
- Node range: echo "0-3" > nid
- Mixed format: echo "0,2-4,7" > nid
- Clear filter: echo > nid (empty string)

The implementation uses nodemask_t for efficient multi-node filtering
and nodelist_parse() for flexible input parsing. Empty input clears
the filter.

Note: Access to nid_mask uses plain load/store without locking because
nodemask_t is too large (128 bytes) for READ_ONCE/WRITE_ONCE. This is
safe for debug use: low-frequency changes and torn reads would only
cause temporary inconsistency in debug output.

Signed-off-by: Zhen Ni <zhen.ni@easystack.cn>
---

Changes in v4:
- Remove "-1" support, use empty string to clear filter
- Use strncpy_from_user() instead of copy_from_user()
- Add concurrency safety documentation for nid_mask access
- Rename fops to page_owner_nid_filter_fops for consistency

Changes in v3:
- Remove READ_ONCE/WRITE_ONCE for nodemask_t (fixes compilation errors)
  * nodemask_t is a large structure (128 bytes) that triggers compile-time asserts
  * Direct assignment is safe for this use case
- Add comment explaining input length calculation formula
  * 6 bytes = ",NNNNN" (comma + 5-digit node number)
- Simplify "-1" check using kstrtoint() instead of dual strcmp()
- Move nodemask_t mask read outside PFN iteration loop for performance
  * Avoids 128-byte structure copy on each iteration

Changes in v2:
- Use nodemask_t instead of int to support multiple nodes
- Implement nodelist_parse() to support flexible input formats
  * Single node: "0", "2"
  * Multiple nodes: "0,2,3"
  * Ranges: "0-3"
  * Mixed: "0,2-4,7"
- Use %*pbl format for output (e.g., "0-2", "0,2-4,7")
- Use dynamic memory allocation (kmalloc) to handle variable-length input
- Follow cpuset's max_write_len pattern: (100 + 6 * MAX_NUMNODES)
---
 mm/page_owner.c | 87 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 87 insertions(+)

diff --git a/mm/page_owner.c b/mm/page_owner.c
index 28766c854d02..68dacf01c822 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -67,10 +67,18 @@ static const char * const page_owner_print_mode_strings[] = {
 
 struct page_owner_filter {
 	enum page_owner_print_mode print_mode;
+	/*
+	 * Access uses plain load/store without locking.
+	 * nodemask_t is too large (128 bytes) for READ_ONCE/WRITE_ONCE.
+	 * Safe for debug use: low-frequency changes, torn reads only cause
+	 * temporary inconsistency in debug output.
+	 */
+	nodemask_t nid_mask;
 };
 
 static struct page_owner_filter owner_filter = {
 	.print_mode = PAGE_OWNER_PRINT_FULL_STACK,
+	.nid_mask = NODE_MASK_NONE,
 };
 
 static bool page_owner_enabled __initdata;
@@ -687,6 +695,7 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
 	struct page_ext *page_ext;
 	struct page_owner *page_owner;
 	depot_stack_handle_t handle;
+	nodemask_t mask;
 
 	if (!static_branch_unlikely(&page_owner_inited))
 		return -EINVAL;
@@ -700,6 +709,8 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
 	while (!pfn_valid(pfn) && (pfn & (MAX_ORDER_NR_PAGES - 1)) != 0)
 		pfn++;
 
+	mask = owner_filter.nid_mask;
+
 	/* Find an allocated page */
 	for (; pfn < max_pfn; pfn++) {
 		/*
@@ -732,6 +743,14 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
 		if (unlikely(!page_ext))
 			continue;
 
+		/* NUMA node filter using bitmask */
+		if (!nodes_empty(mask)) {
+			int nid = page_to_nid(page);
+
+			if (!node_isset(nid, mask))
+				goto ext_put_continue;
+		}
+
 		/*
 		 * Some pages could be missed by concurrent allocation or free,
 		 * because we don't hold the zone lock.
@@ -1054,6 +1073,72 @@ static const struct file_operations page_owner_print_mode_fops = {
 	.llseek = default_llseek,
 };
 
+static ssize_t nid_filter_write(struct file *file,
+				 const char __user *buf,
+				 size_t count, loff_t *ppos)
+{
+	char *kbuf;
+	nodemask_t mask;
+	int ret;
+
+	/*
+	 * Limit input size to handle worst-case nodelist (all nodes).
+	 * Worst case per node: ",NNNNN" (comma + 5-digit node number) = 6 bytes.
+	 * Formula: 100 bytes overhead + 6 * MAX_NUMNODES
+	 */
+	if (count > (100 + 6 * MAX_NUMNODES))
+		return -EINVAL;
+
+	kbuf = kmalloc(count + 1, GFP_KERNEL);
+	if (!kbuf)
+		return -ENOMEM;
+
+	if (strncpy_from_user(kbuf, buf, count) < 0) {
+		ret = -EFAULT;
+		goto out_free;
+	}
+	kbuf[count] = '\0';
+
+	/* Support nodelist format like "0", "0,2", "0-3", or empty to clear */
+	if (nodelist_parse(kbuf, mask)) {
+		ret = -EINVAL;
+		goto out_free;
+	}
+
+	owner_filter.nid_mask = mask;
+	ret = count;
+
+out_free:
+	kfree(kbuf);
+	return ret;
+}
+
+static int nid_filter_show(struct seq_file *m, void *v)
+{
+	nodemask_t mask = owner_filter.nid_mask;
+
+	if (nodes_empty(mask))
+		seq_puts(m, "\n");
+	else
+		seq_printf(m, "%*pbl\n", nodemask_pr_args(&mask));
+
+	return 0;
+}
+
+static int nid_filter_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, nid_filter_show, NULL);
+}
+
+static const struct file_operations page_owner_nid_filter_fops = {
+	.owner		= THIS_MODULE,
+	.open		= nid_filter_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.write		= nid_filter_write,
+	.release	= single_release,
+};
+
 
 static int __init pageowner_init(void)
 {
@@ -1069,6 +1154,8 @@ static int __init pageowner_init(void)
 	filter_dir = debugfs_create_dir("page_owner_filter", NULL);
 	debugfs_create_file("print_mode", 0600, filter_dir, NULL,
 			    &page_owner_print_mode_fops);
+	debugfs_create_file("nid", 0600, filter_dir, NULL,
+			    &page_owner_nid_filter_fops);
 
 	dir = debugfs_create_dir("page_owner_stacks", NULL);
 	debugfs_create_file("show_stacks", 0400, dir,
-- 
2.20.1