Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[David Hildenbrand]

On 24.07.25 09:53, akash.tyagi wrote:
> Currently, THP CMA pages share PCP lists with UNMOVABLE and RECLAIMABLE
> pages. This may result in CMA THP pages being allocated from the PCP
> list for other migratetypes. When this occurs, these pages may fail to
> be isolated, leading to CMA allocation failures when drivers request
> them.

Curious, did you run into that in practice?

Having MIGRATE_CMA pages allocated for unmovable allocations would 
indeed be broken.

But, MIGRATE_PCPTYPES does not include MIGRATE_CMA. So there is also no 
dedicated PCP list for VMA?


In free_unref_folios(), we have

"Non-isolated types over MIGRATE_PCPTYPES get added to the 
MIGRATE_MOVABLE pcp list."

	if (unlikely(migratetype >= MIGRATE_PCPTYPES))
		migratetype = MIGRATE_MOVABLE;

So ... shouldn't that safe us here as well for THPs?

> This patch introduces a dedicated PCP list for the THP CMA migratetype,
> ensuring that CMA THP pages are not mixed with other migratetypes and
> remain available for CMA allocations as intended.
>
> Signed-off-by: akash.tyagi <akash.tyagi@xxxxxxxxxxxx>
> ---
>   include/linux/mmzone.h | 10 ++++++++--
>   mm/page_alloc.c        |  5 +++++
>   2 files changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 283913d42d7b..dd93088ce851 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -696,11 +696,17 @@ enum zone_watermarks {
>   
>   /*
>    * One per migratetype for each PAGE_ALLOC_COSTLY_ORDER. Two additional lists
> - * are added for THP. One PCP list is used by GPF_MOVABLE, and the other PCP list
> - * is used by GFP_UNMOVABLE and GFP_RECLAIMABLE.
> + * are added for THP: one for GFP_MOVABLE, and one for GFP_UNMOVABLE and
> + * GFP_RECLAIMABLE. With CMA enabled, an extra THP PCP list is added for
> + * MIGRATE_CMA, allowing further distinction between MIGRATE_MOVABLE and
> + * MIGRATE_CMA for THP allocations.
>    */
>   #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +#ifdef CONFIG_CMA
> +#define NR_PCP_THP 3
> +#else
>   #define NR_PCP_THP 2
> +#endif
>   #else
>   #define NR_PCP_THP 0
>   #endif
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 2ef3c07266b3..35f8041afbcc 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -519,6 +519,11 @@ static inline unsigned int order_to_pindex(int migratetype, int order)
>   	if (order > PAGE_ALLOC_COSTLY_ORDER) {
>   		VM_BUG_ON(order != HPAGE_PMD_ORDER);
>   
> +#ifdef CONFIG_CMA
> +		if (migratetype == MIGRATE_CMA)
> +			return NR_LOWORDER_PCP_LISTS + 2;
> +#endif
> +
>   		movable = migratetype == MIGRATE_MOVABLE;
>   
>   		return NR_LOWORDER_PCP_LISTS + movable;


--
Cheers,

David / dhildenb