[PATCH v5 4/6] mm: migrate: support poisoned recover from migrate folio

[Kefeng Wang <wangkefeng.wang@huawei.com>]

The folio migration is widely used in kernel, memory compaction, memory
hotplug, soft offline page, numa balance, memory demote/promotion, etc,
but once access a poisoned source folio when migrating, the kerenl will
panic.

There is a mechanism in the kernel to recover from uncorrectable memory
errors, ARCH_HAS_COPY_MC, which is already used in other core-mm paths,
eg, CoW, khugepaged, coredump, ksm copy, see copy_mc_to_{user,kernel},
copy_mc_{user_}highpage callers.

In order to support poisoned folio copy recover from migrate folio, we
chose to make folio migration tolerant of memory failures and return
error for folio migration, because folio migration is no guarantee
of success, this could avoid the similar panic shown below.

  CPU: 1 PID: 88343 Comm: test_softofflin Kdump: loaded Not tainted 6.6.0
  pc : copy_page+0x10/0xc0
  lr : copy_highpage+0x38/0x50
  ...
  Call trace:
   copy_page+0x10/0xc0
   folio_copy+0x78/0x90
   migrate_folio_extra+0x54/0xa0
   move_to_new_folio+0xd8/0x1f0
   migrate_folio_move+0xb8/0x300
   migrate_pages_batch+0x528/0x788
   migrate_pages_sync+0x8c/0x258
   migrate_pages+0x440/0x528
   soft_offline_in_use_page+0x2ec/0x3c0
   soft_offline_page+0x238/0x310
   soft_offline_page_store+0x6c/0xc0
   dev_attr_store+0x20/0x40
   sysfs_kf_write+0x4c/0x68
   kernfs_fop_write_iter+0x130/0x1c8
   new_sync_write+0xa4/0x138
   vfs_write+0x238/0x2d8
   ksys_write+0x74/0x110

Note, folio copy is moved in the begin of the __migrate_folio(), which
could simplify the error handling since there is no turning back if
folio_migrate_mapping() return success, the downside is the folio copied
even though folio_migrate_mapping() return fail, an optimization is to
check whether source folio does not have extra refs before we do folio
copy.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
---
 mm/migrate.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index e97fbaed564d..f9d700d82ea9 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -668,16 +668,24 @@ static int __migrate_folio(struct address_space *mapping, struct folio *dst,
 			   struct folio *src, void *src_private,
 			   enum migrate_mode mode)
 {
-	int rc;
+	int rc, expected_count = folio_expected_refs(mapping, src);
+
+	/* Check whether src does not have extra refs before we do more work */
+	if (folio_ref_count(src) != expected_count)
+		return -EAGAIN;
+
+	rc = folio_mc_copy(dst, src);
+	if (unlikely(rc))
+		return rc;
 
-	rc = folio_migrate_mapping(mapping, dst, src, 0);
+	rc = __folio_migrate_mapping(mapping, dst, src, expected_count);
 	if (rc != MIGRATEPAGE_SUCCESS)
 		return rc;
 
 	if (src_private)
 		folio_attach_private(dst, folio_detach_private(src));
 
-	folio_migrate_copy(dst, src);
+	folio_migrate_flags(dst, src);
 	return MIGRATEPAGE_SUCCESS;
 }
 
-- 
2.27.0