Re: [RFC v2 4/9] khugepaged: generalize alloc_charge_folio for mTHP support

[Ryan Roberts]

On 11/02/2025 00:30, Nico Pache wrote:
> alloc_charge_folio allocates the new folio for the khugepaged collapse.
> Generalize the order of the folio allocations to support future mTHP
> collapsing.
> 
> No functional changes in this patch.
> 
> Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
> ---
>  mm/khugepaged.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index c834ea842847..0cfcdc11cabd 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1074,14 +1074,14 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
>  }
>  
>  static int alloc_charge_folio(struct folio **foliop, struct mm_struct *mm,
> -			      struct collapse_control *cc)
> +			      struct collapse_control *cc, int order)
>  {
>  	gfp_t gfp = (cc->is_khugepaged ? alloc_hugepage_khugepaged_gfpmask() :
>  		     GFP_TRANSHUGE);
>  	int node = khugepaged_find_target_node(cc);
>  	struct folio *folio;
>  
> -	folio = __folio_alloc(gfp, HPAGE_PMD_ORDER, node, &cc->alloc_nmask);
> +	folio = __folio_alloc(gfp, order, node, &cc->alloc_nmask);
>  	if (!folio) {
>  		*foliop = NULL;
>  		count_vm_event(THP_COLLAPSE_ALLOC_FAILED);

Stats management is different for PMD-sized THP vs mTHP. All the PMD-sized THP
stats continue to be accumulated in /proc/meminfo (or whatever its called).
Other THP sizes are not accounted here. All mTHP sizes (*including* PMD-sized)
should accounted in
/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/stats/*. There is a file
for each stat.

We decided to do it this way for fear of breaking unenlightened user space that
only understands PMD-sized THP.

You can find the mTHP stats machinery at count_mthp_stat().

> @@ -1125,7 +1125,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>  	 */
>  	mmap_read_unlock(mm);
>  
> -	result = alloc_charge_folio(&folio, mm, cc);
> +	result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER);
>  	if (result != SCAN_SUCCEED)
>  		goto out_nolock;
>  
> @@ -1851,7 +1851,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>  	VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);
>  	VM_BUG_ON(start & (HPAGE_PMD_NR - 1));
>  
> -	result = alloc_charge_folio(&new_folio, mm, cc);
> +	result = alloc_charge_folio(&new_folio, mm, cc, HPAGE_PMD_ORDER);
>  	if (result != SCAN_SUCCEED)
>  		goto out;
>