[PATCH v2 4/6] mm/page_owner: record the NUMA node in the scan cursor

[Zhen Ni]

Compute the page's NUMA node in page_owner_next_eligible() while the
page is still valid, and store it in scan->nid.

The debugfs nid filter reads scan->nid instead of recomputing it, and a
BPF program iterating the cursor reads the node straight from scan->nid.

Resolve nid at scan time to avoid dereferencing a possibly-unmapped page.
Keep it in the transient scan cursor rather than struct page_owner, so
nid does not add per-page memory overhead.

Signed-off-by: Zhen Ni <zhen.ni@xxxxxxxxxxxx>
---
Changes in v2:
- Removes the v1 get_nid() kfunc. The node is now stored in scan->nid.

v1: https://lore.kernel.org/linux-mm/20261009112308.240769-5-zhen.ni@xxxxxxxxxxxx/
---
 mm/page_owner.c      | 28 ++++++++++++++++------------
 mm/page_owner.h      |  1 +
 mm/page_owner_iter.c |  5 ++++-
 3 files changed, 21 insertions(+), 13 deletions(-)

diff --git a/mm/page_owner.c b/mm/page_owner.c
index 617346a4ab22..113e4e7e0b3a 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -711,6 +711,19 @@ void __dump_page_owner(const struct page *page)
 	page_ext_put(page_ext);
 }
 
+/*
+ * NUMA node of @page, bypassing PF_POISONED_CHECK() in page_to_nid() to
+ * avoid VM_BUG_ON on poisoned pages; -1 for poisoned pages.
+ */
+static int page_owner_get_nid(struct page *page)
+{
+	memdesc_flags_t page_flags = READ_ONCE(page->flags);
+
+	if (page_flags.f == PAGE_POISON_PATTERN)
+		return -1;
+	return memdesc_nid(&page_flags);
+}
+
 /*
  * Advance @scan to the next page eligible for page_owner output.
  * On a hit, fill scan->pfn/page/po_snap and return true; the caller
@@ -782,6 +795,7 @@ bool page_owner_next_eligible(struct page_owner_scan *scan)
 		if (!scan->po_snap.handle)
 			goto ext_put_continue;
 
+		scan->nid = page_owner_get_nid(page);
 		page_ext_put(page_ext);
 		return true;
 
@@ -810,18 +824,8 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
 	/* Find an allocated page */
 	while (page_owner_next_eligible(&scan)) {
 		if (state->nid_filter_enabled) {
-			int nid;
-			memdesc_flags_t page_flags =
-				READ_ONCE(scan.page->flags);
-
-			/*
-			 * Bypass PF_POISONED_CHECK() in page_to_nid() to avoid
-			 * VM_BUG_ON when accessing poisoned pages.
-			 */
-			if (page_flags.f == PAGE_POISON_PATTERN)
-				goto skip_continue;
-			nid = memdesc_nid(&page_flags);
-			if (!node_isset(nid, state->nid_filter))
+			if (scan.nid < 0 ||
+			    !node_isset(scan.nid, state->nid_filter))
 				goto skip_continue;
 		}
 
diff --git a/mm/page_owner.h b/mm/page_owner.h
index fda83be69275..e8e52d22abed 100644
--- a/mm/page_owner.h
+++ b/mm/page_owner.h
@@ -36,6 +36,7 @@ struct page_owner_scan {
 	unsigned long pfn;
 	struct page *page;
 	struct page_owner po_snap;
+	int nid;
 };
 
 bool page_owner_next_eligible(struct page_owner_scan *scan);
diff --git a/mm/page_owner_iter.c b/mm/page_owner_iter.c
index ac7e69ba0cef..a2b90c874852 100644
--- a/mm/page_owner_iter.c
+++ b/mm/page_owner_iter.c
@@ -13,10 +13,12 @@ struct bpf_iter__page_owner {
 	__bpf_md_ptr(struct page *, page);
 	__bpf_md_ptr(struct page_owner *, po);
 	u64 pfn;
+	int nid;
 };
 
 DEFINE_BPF_ITER_FUNC(page_owner, struct bpf_iter_meta *meta,
-		     struct page *page, struct page_owner *po, u64 pfn)
+		     struct page *page, struct page_owner *po, u64 pfn,
+		     int nid)
 
 static void *page_owner_seq_start(struct seq_file *seq, loff_t *posp)
 {
@@ -54,6 +56,7 @@ static int page_owner_prog_seq_show(struct bpf_prog *prog,
 		/* @po: hit-time snapshot, no RCU window needed. */
 		ctx.po = &scan->po_snap;
 		ctx.pfn = scan->pfn;
+		ctx.nid = scan->nid;
 	}
 
 	return bpf_iter_run_prog(prog, &ctx);
-- 
2.20.1