[PATCH v4 3/4] 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 at nvidia.com>
Acked-by: Peter Xu <peterx at redhat.com>
Reviewed-by: "Huang, Ying" <ying.huang at intel.com>
Reported-by: "Huang, Ying" <ying.huang at intel.com>
Fixes: 8c3328f1f36a ("mm/migrate: migrate_vma() unmap page from vma while collecting pages")
Cc: stable at vger.kernel.org

---

Changes for v4:

 - Added Reviewed-by

Changes for v3:

 - Defer TLB flushing
 - Split a TLB flushing fix into a separate change.

Changes for v2:

 - Fixed up Reported-by tag.
 - Added Peter's Acked-by.
 - Atomically read and clear the pte to prevent the dirty bit getting
   set after reading it.
 - Added fixes tag
---
 mm/migrate_device.c |  9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 4cc849c..dbf6c7a 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>
@@ -196,7 +197,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
 			flush_cache_page(vma, addr, pte_pfn(*ptep));
 			anon_exclusive = PageAnon(page) && PageAnonExclusive(page);
 			if (anon_exclusive) {
-				ptep_clear_flush(vma, addr, ptep);
+				pte = ptep_clear_flush(vma, addr, ptep);
 
 				if (page_try_share_anon_rmap(page)) {
 					set_pte_at(mm, addr, ptep, pte);
@@ -206,11 +207,15 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
 					goto next;
 				}
 			} else {
-				ptep_get_and_clear(mm, addr, ptep);
+				pte = ptep_get_and_clear(mm, addr, ptep);
 			}
 
 			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(
-- 
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