+ mm-migrate_device-clear-stale-mapping-after-freeing-swapcache.patch added to mm-new branch

The patch titled
     Subject: mm/migrate_device: clear stale mapping after freeing swapcache
has been added to the -mm mm-new branch.  Its filename is
     mm-migrate_device-clear-stale-mapping-after-freeing-swapcache.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-migrate_device-clear-stale-mapping-after-freeing-swapcache.patch

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

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

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------------------------------------------------------
From: Arvind Yadav <[email protected]>
Subject: mm/migrate_device: clear stale mapping after freeing swapcache
Date: Tue, 28 Jul 2026 11:58:32 +0530

__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.

Link: https://lore.kernel.org/[email protected]
Fixes: df263d9a7dff ("mm/migrate_device: try to handle swapcache pages")
Signed-off-by: Arvind Yadav <[email protected]>
Reviewed-by: Zi Yan <[email protected]>
Reviewed-by: Balbir Singh <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Matthew Brost <[email protected]>
Cc: Joshua Hahn <[email protected]>
Cc: Rakie Kim <[email protected]>
Cc: Byungchul Park <[email protected]>
Cc: Gregory Price <[email protected]>
Cc: Ying Huang <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
---

 mm/migrate_device.c |   13 +++++++++++++
 1 file changed, 13 insertions(+)

--- a/mm/migrate_device.c~mm-migrate_device-clear-stale-mapping-after-freeing-swapcache
+++ a/mm/migrate_device.c
@@ -1193,6 +1193,13 @@ static void __migrate_device_pages(unsig
 							 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(unsig
 			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;
_

Patches currently in -mm which might be from [email protected] are

mm-migrate_device-clear-stale-mapping-after-freeing-swapcache.patch