[PATCH 1/2] mm: Retry migration earlier upon refcount mismatch

[David Hildenbrand]

On 09.08.24 12:31, Dev Jain wrote:
> 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 at 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 :)

>   		__migrate_folio_record(dst, old_page_state, anon_vma);
>   		return MIGRATEPAGE_UNMAP;
>   	}

-- 
Cheers,

David / dhildenb