FAILED: patch "[PATCH] mm/migrate_device: clear stale mapping after freeing" failed to apply to 6.18-stable tree

The patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x 34a00895d032a414830d41106a09329ae6c251b6
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026090342-staging-boned-79f6@gregkh' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 34a00895d032a414830d41106a09329ae6c251b6 Mon Sep 17 00:00:00 2001
From: Arvind Yadav <[email protected]>
Date: Tue, 28 Jul 2026 11:58:32 +0530
Subject: [PATCH] mm/migrate_device: clear stale mapping after freeing
swapcache

__migrate_device_pages() reads the folio mapping before calling folio_free_swap(). When folio_free_swap() succeeds, the folio is removed from the swap cache, but the saved mapping still points to swap_space.

Passing the stale mapping to folio_migrate_mapping() makes it use the mapped-folio path for a folio that is no longer in swapcache. It can then operate on swap_space.i_pages with invalid reference accounting, eventually triggering a folio reference count BUG.

After a successful split, nr still contains the number of pages in the original large folio, although each resulting page is now a separate order-0 folio. Reset nr to 1 so each split folio is processed separately, including its own swapcache removal and mapping lookup.

Refresh the saved mapping after folio_free_swap() so the current folio state is used during migration.

index 18d097c38853..9a346162c688 100644
@@ -1193,6 +1193,13 @@ static void __migrate_device_pages(unsigned long *src_pfns,
 							 MIGRATE_PFN_COMPOUND);
 					goto next;
 				}
+
+				/*
+				 * reset nr so that only first after-split folio
+				 * is processed below
+				 */
+				VM_WARN_ON_ONCE(folio_test_large(folio));
+				nr = 1;
 			} else if ((src_pfns[i] & MIGRATE_PFN_MIGRATE) &&
 				(dst_pfns[i] & MIGRATE_PFN_COMPOUND) &&
 				!(src_pfns[i] & MIGRATE_PFN_COMPOUND)) {

@@ -1232,6 +1239,12 @@ static void __migrate_device_pages(unsigned long *src_pfns,
 			folio = page_folio(migrate_pfn_to_page(src_pfns[i+j]));
 			newfolio = page_folio(migrate_pfn_to_page(dst_pfns[i+j]));
 
+			/*
+			 * folio_free_swap() removed the folio from the swap
+			 * cache. Refresh the saved mapping before migration.
+			 */
+			mapping = folio_mapping(folio);
+
 			r = folio_migrate_mapping(mapping, newfolio, folio, extra_cnt);
 			if (r)
 				src_pfns[i+j] &= ~MIGRATE_PFN_MIGRATE;