On 9 Aug 2024, at 9:47, David Hildenbrand wrote:

On 09.08.24 12:31, Dev Jain wrote:

quoted

As already being done in __migrate_folio(), wherein we backoff if the
folio refcount is wrong, make this check during the unmapping phase, upon
the failure of which, the original state of the PTEs will be restored and
the folio lock will be dropped via migrate_folio_undo_src(), any racing
thread will make progress and migration will be retried.

Signed-off-by: Dev Jain <dev.jain@arm.com>
---
  mm/migrate.c | 9 +++++++++
  1 file changed, 9 insertions(+)

diff --git a/mm/migrate.c b/mm/migrate.c
index e7296c0fb5d5..477acf996951 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c

@@ -1250,6 +1250,15 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
  	}
   	if (!folio_mapped(src)) {
+		/*
+		 * Someone may have changed the refcount and maybe sleeping
+		 * on the folio lock. In case of refcount mismatch, bail out,
+		 * let the system make progress and retry.
+		 */
+		struct address_space *mapping = folio_mapping(src);
+
+		if (folio_ref_count(src) != folio_expected_refs(mapping, src))
+			goto out;

This really seems to be the latest point where we can "easily" back off and unlock the source folio -- in this function :)

I wonder if we should be smarter in the migrate_pages_batch() loop when we start the actual migrations via migrate_folio_move(): if we detect that a folio has unexpected references *and* it has waiters (PG_waiters), back off then and retry the folio later. If it only has unexpected references, just keep retrying: no waiters -> nobody is waiting for the lock to make progress.

For example, when migrate_folio_move() fails with -EAGAIN, check if there are waiters (PG_waiter?) and undo+unlock to try again later.

But I'm not really a migration expert, so only my 2 cents :)

Sounds reasonable to me. Also add Ying (the author of batch page migration).

--
Best Regards,
Yan, Zi