[PATCH 1/2] mm/migrate_device.c: Copy pte dirty bit to page

[Alistair Popple]

migrate_vma_setup() has a fast path in migrate_vma_collect_pmd() that
installs migration entries directly if it can lock the migrating page.
When removing a dirty pte the dirty bit is supposed to be carried over
to the underlying page to prevent it being lost.

Currently migrate_vma_*() can only be used for private anonymous
mappings. That means loss of the dirty bit usually doesn't result in
data loss because these pages are typically not file-backed. However
pages may be backed by swap storage which can result in data loss if an
attempt is made to migrate a dirty page that doesn't yet have the
PageDirty flag set.

In this case migration will fail due to unexpected references but the
dirty pte bit will be lost. If the page is subsequently reclaimed data
won't be written back to swap storage as it is considered uptodate,
resulting in data loss if the page is subsequently accessed.

Prevent this by copying the dirty bit to the page when removing the pte
to match what try_to_migrate_one() does.

Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx>
Reported-by: Peter Xu <peterx@xxxxxxxxxx>
---
 mm/migrate_device.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 27fb37d..d38f8a6 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -7,6 +7,7 @@
 #include <linux/export.h>
 #include <linux/memremap.h>
 #include <linux/migrate.h>
+#include <linux/mm.h>
 #include <linux/mm_inline.h>
 #include <linux/mmu_notifier.h>
 #include <linux/oom.h>
@@ -211,6 +212,10 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
 
 			migrate->cpages++;
 
+			/* Set the dirty flag on the folio now the pte is gone. */
+			if (pte_dirty(pte))
+				folio_mark_dirty(page_folio(page));
+
 			/* Setup special migration page table entry */
 			if (mpfn & MIGRATE_PFN_WRITE)
 				entry = make_writable_migration_entry(

base-commit: ffcf9c5700e49c0aee42dcba9a12ba21338e8136
-- 
git-series 0.9.1