On 1/15/26 10:19, David Hildenbrand (Red Hat) wrote:

Let's update the balloon page references, the balloon page list, the
BALLOON_MIGRATE counter and the isolated-pages counter in
balloon_page_migrate(), after letting the balloon->migratepage()
callback deal with the actual inflation+deflation.

Note that we now perform the balloon list modifications outside of any
implementation-specific locks: which is fine, there is nothing special
about these page actions that the lock would be protecting.

The old page is already no longer in the list (isolated) and the new page
is not yet in the list.

Let's use -ENOENT to communicate the special "inflation of new page
failed after already deflating the old page" to balloon_page_migrate() so
it can handle it accordingly.

While at it, rename balloon->b_dev_info to make it match the other
functions. Also, drop the comment above balloon_page_migrate(), which
seems unnecessary.

Signed-off-by: David Hildenbrand (Red Hat) <david@kernel.org>
---

Andrew, the following on top:

 From 4c8b4f0aba5859a4ec71c7449a98b10e0547237f Mon Sep 17 00:00:00 2001
From: "David Hildenbrand (Red Hat)" <david@kernel.org>
Date: Mon, 19 Jan 2026 23:20:41 +0100
Subject: [PATCH] fixup: mm/balloon_compaction: centralize basic page migration
  handling

Remove newline, talk about "page" instead of "old page" and avoid the
switch.

Signed-off-by: David Hildenbrand (Red Hat) <david@kernel.org>
---
  mm/balloon_compaction.c | 22 +++++++---------------
  1 file changed, 7 insertions(+), 15 deletions(-)

diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
index 5444c61bb9e76..b859411811d0b 100644
--- a/mm/balloon_compaction.c
+++ b/mm/balloon_compaction.c

@@ -247,29 +247,21 @@ static int balloon_page_migrate(struct page *newpage, struct page *page,
  		return -EAGAIN;
  
  	rc = b_dev_info->migratepage(b_dev_info, newpage, page, mode);
-	switch (rc) {
-	case 0:
-		spin_lock_irqsave(&b_dev_info->pages_lock, flags);
+	if (rc < 0 && rc != -ENOENT)
+		return rc;
  
+	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
+	if (!rc) {
  		/* Insert the new page into the balloon list. */
  		get_page(newpage);
-
  		balloon_page_insert(b_dev_info, newpage);
-		__count_vm_event(BALLOON_MIGRATE);
-		break;
-	case -ENOENT:
-		spin_lock_irqsave(&b_dev_info->pages_lock, flags);
-
-		/* Old page was deflated but new page not inflated. */
-		__count_vm_event(BALLOON_DEFLATE);
-		break;
-	default:
-		return rc;
  	}
-
  	b_dev_info->isolated_pages--;
  	spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
  
+	/* If -ENOENT, page was deflated but new page not inflated. */
+	__count_vm_event(rc ? BALLOON_DEFLATE : BALLOON_MIGRATE);
+
  	/* Free the now-deflated page we isolated in balloon_page_isolate(). */
  	balloon_page_finalize(page);
  	put_page(page);

-- 
2.52.0


-- 
Cheers

David