Dev Jain dev.jain@arm.com writes:
On 8/12/24 11:04, Huang, Ying wrote:
Hi, Dev,
Dev Jain dev.jain@arm.com writes:
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;Do you have some test results for this? For example, after applying the patch, the migration success rate increased XX%, etc.
Noting that the migration selftest is operating on a single page, before the patch, the test fails on shared-anon mappings on an average of 10 iterations of move_pages(), and after applying the patch it fails on average of 100 iterations, which makes sense because the unmapping() will get retried 3 + 7 = 10 times.
Thanks! What is the test results for
https://lore.kernel.org/all/20240801081657.1386743-1-dev.jain@arm.com/
?
My understanding for this issue is that the migration success rate can increase if we undo all changes before retrying. This is the current behavior for sync migration, but not for async migration. If so, we can use migrate_pages_sync() for async migration too to increase success rate? Of course, we need to change the function name and comments.
-- Best Regards, Huang, Ying