The patch titled Subject: vmw_balloon: indicate success when effectively deflating during migration has been added to the -mm mm-hotfixes-unstable branch. Its filename is vmw_balloon-indicate-success-when-effectively-deflating-during-migration.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: David Hildenbrand david@redhat.com Subject: vmw_balloon: indicate success when effectively deflating during migration Date: Tue, 14 Oct 2025 14:44:55 +0200

When migrating a balloon page, we first deflate the old page to then inflate the new page.

However, if inflating the new page succeeded, we effectively deflated the old page, reducing the balloon size.

In that case, the migration actually worked: similar to migrating+ immediately deflating the new page. The old page will be freed back to the buddy.

Right now, the core will leave the page be marked as isolated (as we returned an error). When later trying to putback that page, we will run into the WARN_ON_ONCE() in balloon_page_putback().

That handling was changed in commit 3544c4faccb8 ("mm/balloon_compaction: stop using __ClearPageMovable()"); before that change, we would have tolerated that way of handling it.

To fix it, let's just return 0 in that case, making the core effectively just clear the "isolated" flag + freeing it back to the buddy as if the migration succeeded. Note that the new page will also get freed when the core puts the last reference.

Note that this also makes it all be more consistent: we will no longer unisolate the page in the balloon driver while keeping it marked as being isolated in migration core.

This was found by code inspection.

Link: https://lkml.kernel.org/r/20251014124455.478345-1-david@redhat.com Fixes: 3544c4faccb8 ("mm/balloon_compaction: stop using __ClearPageMovable()") Signed-off-by: David Hildenbrand david@redhat.com Cc: Jerrin Shaji George jerrin.shaji-george@broadcom.com Cc: Broadcom internal kernel review list bcm-kernel-feedback-list@broadcom.com Cc: Arnd Bergmann arnd@arndb.de Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

drivers/misc/vmw_balloon.c | 8 +++----- 1 file changed, 3 insertions(+), 5 deletions(-)

--- a/drivers/misc/vmw_balloon.c~vmw_balloon-indicate-success-when-effectively-deflating-during-migration +++ a/drivers/misc/vmw_balloon.c @@ -1737,7 +1737,7 @@ static int vmballoon_migratepage(struct { unsigned long status, flags; struct vmballoon *b; - int ret; + int ret = 0;

b = container_of(b_dev_info, struct vmballoon, b_dev_info);

@@ -1796,17 +1796,15 @@ static int vmballoon_migratepage(struct * A failure happened. While we can deflate the page we just * inflated, this deflation can also encounter an error. Instead * we will decrease the size of the balloon to reflect the - * change and report failure. + * change. */ atomic64_dec(&b->size); - ret = -EBUSY; } else { /* * Success. Take a reference for the page, and we will add it to * the list after acquiring the lock. */ get_page(newpage); - ret = 0; }

/* Update the balloon list under the @pages_lock */ @@ -1817,7 +1815,7 @@ static int vmballoon_migratepage(struct * If we succeed just insert it to the list and update the statistics * under the lock. */ - if (!ret) { + if (status == VMW_BALLOON_SUCCESS) { balloon_page_insert(&b->b_dev_info, newpage); __count_vm_event(BALLOON_MIGRATE); } _

Patches currently in -mm which might be from david@redhat.com are

vmw_balloon-indicate-success-when-effectively-deflating-during-migration.patch