Re: [PATCH 4/8] migrate_pages: split unmap_and_move() to _unmap() and _move()

[Nathan Chancellor]

Hi Ying,

On Tue, Dec 27, 2022 at 08:28:55AM +0800, Huang Ying wrote:
> This is a preparation patch to batch the folio unmapping and moving.
> 
> In this patch, unmap_and_move() is split to migrate_folio_unmap() and
> migrate_folio_move().  So, we can batch _unmap() and _move() in
> different loops later.  To pass some information between unmap and
> move, the original unused dst->mapping and dst->private are used.
> 
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Yang Shi <shy828301@xxxxxxxxx>
> Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Cc: Oscar Salvador <osalvador@xxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Bharata B Rao <bharata@xxxxxxx>
> Cc: Alistair Popple <apopple@xxxxxxxxxx>
> Cc: haoxin <xhao@xxxxxxxxxxxxxxxxx>
> ---
>  include/linux/migrate.h |   1 +
>  mm/migrate.c            | 162 +++++++++++++++++++++++++++++-----------
>  2 files changed, 121 insertions(+), 42 deletions(-)
> 
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index 3ef77f52a4f0..7376074f2e1e 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -18,6 +18,7 @@ struct migration_target_control;
>   * - zero on page migration success;
>   */
>  #define MIGRATEPAGE_SUCCESS		0
> +#define MIGRATEPAGE_UNMAP		1
>  
>  /**
>   * struct movable_operations - Driver page migration
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 97ea0737ab2b..e2383b430932 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1009,11 +1009,29 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
>  	return rc;
>  }
>  
> -static int __unmap_and_move(struct folio *src, struct folio *dst,
> +static void __migrate_folio_record(struct folio *dst,
> +				   unsigned long page_was_mapped,
> +				   struct anon_vma *anon_vma)
> +{
> +	dst->mapping = (struct address_space *)anon_vma;
> +	dst->private = (void *)page_was_mapped;
> +}
> +
> +static void __migrate_folio_extract(struct folio *dst,
> +				   int *page_was_mappedp,
> +				   struct anon_vma **anon_vmap)
> +{
> +	*anon_vmap = (struct anon_vma *)dst->mapping;
> +	*page_was_mappedp = (unsigned long)dst->private;
> +	dst->mapping = NULL;
> +	dst->private = NULL;
> +}

This patch as commit 42871c600cad ("migrate_pages: split
unmap_and_move() to _unmap() and _move()") in next-20230105 causes the
following error with clang when CONFIG_RANDSTRUCT is enabled, which is
the case with allmodconfig:

  ../mm/migrate.c:1041:15: error: casting from randomized structure pointer type 'struct address_space *' to 'struct anon_vma *'
          *anon_vmap = (struct anon_vma *)dst->mapping;
                       ^
  1 error generated.

With GCC, there is only a note:

  ../mm/migrate.c: In function '__migrate_folio_extract':
  ../mm/migrate.c:1041:20: note: randstruct: casting between randomized structure pointer types (ssa): 'struct anon_vma' and 'struct address_space'

   1041 |         *anon_vmap = (struct anon_vma *)dst->mapping;
        |         ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Kees has done fixes for warnings and errors like this in the past (I
just ran

  $ git log -p --grep='randomized structure pointer type'

to find them) but I did not see any that would seem appropriate here
hence just the report :)

Cheers,
Nathan