Re: [PATCH v3 21/26] mm/page_alloc: implement FREETYPE_UNMAPPED allocations

> diff --git a/mm/compaction.c b/mm/compaction.c
> index 67b01af024e17..c9eb3947ffc79 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1393,6 +1393,9 @@ static bool suitable_migration_source(struct compact_control *cc,
>         block_ft = get_pageblock_freetype(page);
>         block_mt = free_to_migratetype(block_ft);
>
> +       if (freetype_unmapped(get_pageblock_freetype(page)))
> +               return false;
> +
>         /*
>          * CMA pages can only be taken by ALLOC_CMA requests. For anybody
>          * else, vacating a CMA block consumes free pages the caller
> @@ -1444,6 +1447,9 @@ static bool suitable_migration_target(struct compact_control *cc,
>                         return false;
>         }
>
> +       if (freetype_unmapped(get_pageblock_freetype(page)))
> +               return false;
> +

Do we also want to add safety checks in the page migration code? IIUC,
if we miss a check in the compaction code and try to compact an
unmapped pageblock the kernel will crash with a #PF in supervisor
mode.

>         if (cc->ignore_block_suitable)
>                 return true;
>