Re: [PATCH 06/11] migrate: Remove call to ->writepage

["Zi Yan" <ziy@nvidia.com>]

On Fri Mar 7, 2025 at 8:54 AM EST, Matthew Wilcox (Oracle) wrote:
> The writepage callback is going away; filesystems must implement
> migrate_folio or else dirty folios will not be migratable.

What is the impact of this? Are there any filesystem that has
a_ops->writepage() without migrate_folio()? I wonder if it could make
the un-migratable problem worse[1] when such FS exists.

[1] https://lore.kernel.org/linux-mm/882b566c-34d6-4e68-9447-6c74a0693f18@redhat.com/

>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  mm/migrate.c | 57 ++++------------------------------------------------
>  1 file changed, 4 insertions(+), 53 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index c0adea67cd62..3d1d9d49fb8e 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -944,67 +944,18 @@ int filemap_migrate_folio(struct address_space *mapping,
>  }
>  EXPORT_SYMBOL_GPL(filemap_migrate_folio);
>  
> -/*
> - * Writeback a folio to clean the dirty state
> - */
> -static int writeout(struct address_space *mapping, struct folio *folio)
> -{
> -	struct writeback_control wbc = {
> -		.sync_mode = WB_SYNC_NONE,
> -		.nr_to_write = 1,
> -		.range_start = 0,
> -		.range_end = LLONG_MAX,
> -		.for_reclaim = 1
> -	};
> -	int rc;
> -
> -	if (!mapping->a_ops->writepage)
> -		/* No write method for the address space */
> -		return -EINVAL;
> -
> -	if (!folio_clear_dirty_for_io(folio))
> -		/* Someone else already triggered a write */
> -		return -EAGAIN;
> -
> -	/*
> -	 * A dirty folio may imply that the underlying filesystem has
> -	 * the folio on some queue. So the folio must be clean for
> -	 * migration. Writeout may mean we lose the lock and the
> -	 * folio state is no longer what we checked for earlier.
> -	 * At this point we know that the migration attempt cannot
> -	 * be successful.
> -	 */
> -	remove_migration_ptes(folio, folio, 0);
> -
> -	rc = mapping->a_ops->writepage(&folio->page, &wbc);
> -
> -	if (rc != AOP_WRITEPAGE_ACTIVATE)
> -		/* unlocked. Relock */
> -		folio_lock(folio);
> -
> -	return (rc < 0) ? -EIO : -EAGAIN;
> -}
> -
>  /*
>   * Default handling if a filesystem does not provide a migration function.
>   */
>  static int fallback_migrate_folio(struct address_space *mapping,
>  		struct folio *dst, struct folio *src, enum migrate_mode mode)
>  {
> -	if (folio_test_dirty(src)) {
> -		/* Only writeback folios in full synchronous migration */
> -		switch (mode) {
> -		case MIGRATE_SYNC:
> -			break;
> -		default:
> -			return -EBUSY;
> -		}
> -		return writeout(mapping, src);
> -	}

Now fallback_migrate_folio() no longer writes out page for FS, so it is
the responsibilty of migrate_folio()?

+Joanne, since she is touching the above code in the FUSE temp page removal
patchset.

> +	if (folio_test_dirty(src))
> +		return -EBUSY;
>  
>  	/*
> -	 * Buffers may be managed in a filesystem specific way.
> -	 * We must have no buffers or drop them.
> +	 * Filesystem may have private data at folio->private that we
> +	 * can't migrate automatically.
>  	 */
>  	if (!filemap_release_folio(src, GFP_KERNEL))
>  		return mode == MIGRATE_SYNC ? -EAGAIN : -EBUSY;


-- 
Best Regards,
Yan, Zi