Re: [PATCH v1 mm-hotfixes] mm/zsmalloc: do not pass __GFP_MOVABLE if CONFIG_COMPACTION=n

[Harry Yoo <harry.yoo@oracle.com>]

On Fri, Jul 04, 2025 at 07:30:53PM +0900, Harry Yoo wrote:
> Commit 48b4800a1c6a ("zsmalloc: page migration support") added support
> for migrating zsmalloc pages using the movable_operations migration
> framework. However, the commit did not take into account that zsmalloc
> supports migration only when CONFIG_COMPACTION is enabled.
> Tracing shows that zsmalloc was still passing the __GFP_MOVABLE flag
> even when compaction is not supported.
> 
> This can result in unmovable pages being allocated from movable page
> blocks (even without stealing page blocks), ZONE_MOVABLE and CMA area.
> 
> Clear the __GFP_MOVABLE flag when !IS_ENABLED(CONFIG_COMPACTION).
> 
> Cc: stable@vger.kernel.org
> Fixes: 48b4800a1c6a ("zsmalloc: page migration support")
> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> ---

Possible user visible effects:
- Some ZONE_MOVABLE memory can be not actually movable
- CMA allocation can fail because of this
- Increased memory fragmentation due to ignoring the page mobility
  grouping feature  

I'm not really sure who uses kernels without compaction support, though :(

>  mm/zsmalloc.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 999b513c7fdf..f3e2215f95eb 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -1043,6 +1043,9 @@ static struct zspage *alloc_zspage(struct zs_pool *pool,
>  	if (!zspage)
>  		return NULL;
>  
> +	if (!IS_ENABLED(CONFIG_COMPACTION))
> +		gfp &= ~__GFP_MOVABLE;
> +
>  	zspage->magic = ZSPAGE_MAGIC;
>  	zspage->pool = pool;
>  	zspage->class = class->index;
> -- 
> 2.43.0
> 

-- 
Cheers,
Harry / Hyeonggon