Re: [PATCH v7 mm-new 03/10] mm: thp: decouple THP allocation between swap and page fault paths

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Wed, Sep 10, 2025 at 10:44:40AM +0800, Yafang Shao wrote:
> The new BPF capability enables finer-grained THP policy decisions by
> introducing separate handling for swap faults versus normal page faults.
>
> As highlighted by Barry:
>
>   We’ve observed that swapping in large folios can lead to more
>   swap thrashing for some workloads- e.g. kernel build. Consequently,
>   some workloads might prefer swapping in smaller folios than those
>   allocated by alloc_anon_folio().
>
> While prtcl() could potentially be extended to leverage this new policy,
> doing so would require modifications to the uAPI.
>
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>

Other than nits, these seems fine, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> Cc: Barry Song <21cnbao@gmail.com>
> ---
>  include/linux/huge_mm.h | 3 ++-
>  mm/huge_memory.c        | 2 +-
>  mm/memory.c             | 2 +-
>  3 files changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index f72a5fd04e4f..b9742453806f 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -97,9 +97,10 @@ extern struct kobj_attribute thpsize_shmem_enabled_attr;
>
>  enum tva_type {
>  	TVA_SMAPS,		/* Exposing "THPeligible:" in smaps. */
> -	TVA_PAGEFAULT,		/* Serving a page fault. */
> +	TVA_PAGEFAULT,		/* Serving a non-swap page fault. */
>  	TVA_KHUGEPAGED,		/* Khugepaged collapse. */
>  	TVA_FORCED_COLLAPSE,	/* Forced collapse (e.g. MADV_COLLAPSE). */
> +	TVA_SWAP,		/* Serving a swap */

Serving a swap what? :) I think TVA_SWAP_PAGEFAULT would be better here right?
And 'serving a swap page fault'.

>  };
>
>  #define thp_vma_allowable_order(vma, vm_flags, type, order) \
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 26cedfcd7418..523153d21a41 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -103,7 +103,7 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>  					 unsigned long orders)
>  {
>  	const bool smaps = type == TVA_SMAPS;
> -	const bool in_pf = type == TVA_PAGEFAULT;
> +	const bool in_pf = (type == TVA_PAGEFAULT || type == TVA_SWAP);
>  	const bool forced_collapse = type == TVA_FORCED_COLLAPSE;
>  	unsigned long supported_orders;
>
> diff --git a/mm/memory.c b/mm/memory.c
> index d9de6c056179..d8819cac7930 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4515,7 +4515,7 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf)
>  	 * Get a list of all the (large) orders below PMD_ORDER that are enabled
>  	 * and suitable for swapping THP.
>  	 */
> -	orders = thp_vma_allowable_orders(vma, vma->vm_flags, TVA_PAGEFAULT,
> +	orders = thp_vma_allowable_orders(vma, vma->vm_flags, TVA_SWAP,
>  					  BIT(PMD_ORDER) - 1);
>  	orders = thp_vma_suitable_orders(vma, vmf->address, orders);
>  	orders = thp_swap_suitable_orders(swp_offset(entry),
> --
> 2.47.3
>