Re: [PATCH v5 32/36] mm/khugepaged: base private node collapse eligiblity on actor/cap bits

[Richard Cheng]

On Mon, Jul 20, 2026 at 03:34:26PM +0800, Gregory Price wrote:
> Collapse can move base pages across nodes.  The khugepaged daemon does
> this transparently, while MADV_COLLAPSE is a userland construct.
> 
> Isolate private nodes from khugepaged exactly like ZONE_DEVICE.
> Never collapse private node folios silently (the owner may not
> support migration).
> 
> MADV_COLLAPSE is user-initiated, so it is allowed on a private node
> opted into CAP_USER_NUMA, consistent with other userspace interfaces.
> 
> folio_allows_collapse() encodes this: false for khugepaged on a private
> node, CAP_USER_NUMA for MADV_COLLAPSE.
> 
> Signed-off-by: Gregory Price <gourry@xxxxxxxxxx>
> ---
>  mm/internal.h   | 26 ++++++++++++++++++++++++++
>  mm/khugepaged.c | 16 +++++++++++++---
>  2 files changed, 39 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index 9621eb127c28c..9dbc5752cd3fe 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -136,6 +136,32 @@ static inline bool folio_allows_numa_balance(struct folio *folio)
>  	       node_allows_numa_balancing(folio_nid(folio));
>  }
>  
> +/*
> + * folio_allows_collapse() - may collapse fold this folio into a THP?
> + * @is_khugepaged: true for the khugepaged, false for MADV_COLLAPSE.
> + *
> + * Collapse is a residency operation gated by the actor calling it:
> + *   - khugepaged never operates on private-node folios (like ZONE_DEVICE)
> + *   - MADV_COLLAPSE is gated by CAP_USER_NUMA
> + *
> + * Never true for ZONE_DEVICE.
> + */
> +static inline bool folio_allows_collapse(struct folio *folio, bool is_khugepaged)
> +{
> +	int nid = folio_nid(folio);
> +
> +	if (folio_is_zone_device(folio))
> +		return false;
> +	if (is_khugepaged)
> +		return !node_is_private(nid);
> +	return node_allows_user_numa(nid);
> +}
> +
> +static inline bool page_allows_collapse(struct page *page, bool is_khugepaged)
> +{
> +	return folio_allows_collapse(page_folio(page), is_khugepaged);
> +}
> +
>  /*
>   * folio_allows_longterm_pin() - may this folio be long-term GUP-pinned?
>   *
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index fb4378cc17b10..5f20839857738 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -700,7 +700,8 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
>  			goto out;
>  		}
>  		page = vm_normal_page(vma, addr, pteval);
> -		if (unlikely(!page) || unlikely(page_is_private_managed(page))) {
> +		if (unlikely(!page) ||
> +		    unlikely(!page_allows_collapse(page, cc->is_khugepaged))) {
>  			result = SCAN_PAGE_NULL;
>  			goto out;
>  		}
> @@ -1241,9 +1242,17 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru
>  	gfp_t gfp = (cc->is_khugepaged ? alloc_hugepage_khugepaged_gfpmask() :
>  		     GFP_TRANSHUGE);
>  	int node = collapse_find_target_node(cc);
> +	unsigned int aflags;
>  	struct folio *folio;
> +	bool allow;
>  
> -	folio = __folio_alloc(gfp, order, node, &cc->alloc_nmask, ALLOC_DEFAULT);
> +	/* Private node access: khugepaged never, madvise with CAP_USER_NUMA */
> +	allow = cc->is_khugepaged ? !node_is_private(node)
> +				  : node_allows_user_numa(node);
> +	aflags = (allow && node_is_private(node)) ?
> +		ALLOC_ZONELIST_PRIVATE : ALLOC_DEFAULT;
> +
> +	folio = __folio_alloc(gfp, order, node, &cc->alloc_nmask, aflags);
>  	if (!folio) {
>  		*foliop = NULL;
>  		if (is_pmd_order(order))
> @@ -1687,7 +1696,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
>  		}
>  
>  		page = vm_normal_page(vma, addr, pteval);
> -		if (unlikely(!page) || unlikely(page_is_private_managed(page))) {
> +		if (unlikely(!page) ||
> +		    unlikely(!page_allows_collapse(page, cc->is_khugepaged))) {

I think collapse_scan_file() need something like folio_allows_collapse() check as well.

--Richard

>  			result = SCAN_PAGE_NULL;
>  			goto out_unmap;
>  		}
> -- 
> 2.53.0-Meta
> 
>