Re: [PATCH v4 5/7] mm/khugepaged: Refactor the PTE state checks into a helper

["Zi Yan" <ziy@nvidia.com>]

On Tue Aug 11, 2026 at 8:48 AM EDT, Nico Pache (Red Hat) wrote:
> For anonymous collapse, the collapse_scan_pmd() and
> __collapse_huge_page_isolate() functions share a large portion of their
> logic. These functions both check the state of the PTEs and verify the
> following:
> 	- max_pte_* values are not exceeded
> 	- uffd is not active
> 	- lazyfree properties
> 	- non-anonymous
>
> Merge these checks into a helper collapse_check_pte() to reduce code
> duplication. We also add a helper struct for this function called
> pte_check_context which allows us to pass the required parameters in a
> clean and elegant manner.
>
> A helper function is also introduced pte_check_fail() to provide a clean
> interface to set the pte_check_context failure results and return
> PTE_CHECK_FAIL state. This helps reduce code duplications across the new
> collapse_check_pte function.
>
> Two slight modifications are done to the original functionality. We now
> warn (instead of crash) if the anon test fails, and we leverage the
> vm_normal_folio function instead of page->folio, this should be
> functionally equivalent.
>
> No other functional changes intended.
>
> This patch is heavily based off work done by Lance Yang, but modified to
> deal with conflicts and feedback received during the review cycle [1].
>
> [1] https://lore.kernel.org/linux-mm/20251008043748.45554-1-lance.yang@linux.dev/
> Suggested-by: David Hildenbrand <david@kernel.org>
> Signed-off-by: Nico Pache (Red Hat) <nico.pache@linux.dev>
> ---
>  mm/khugepaged.c | 298 +++++++++++++++++++++++++++++---------------------------
>  1 file changed, 157 insertions(+), 141 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 90d6e595d282..b7372aba4417 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -65,6 +65,12 @@ enum scan_result {
>  	SCAN_PAGE_DIRTY_OR_WRITEBACK,
>  };
>  
> +enum pte_check_result {
> +	PTE_CHECK_SUCCEED,
> +	PTE_CHECK_FAIL,
> +	PTE_CHECK_CONTINUE,
> +};
> +
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/huge_memory.h>
>  
> @@ -119,6 +125,20 @@ struct collapse_control {
>  	DECLARE_BITMAP(mthp_present_ptes, MAX_PTRS_PER_PTE);
>  };
>  
> +struct pte_check_context {
> +	struct collapse_control *cc;
> +	struct vm_area_struct *vma;
> +	unsigned int order;
> +	struct folio *folio;
> +	int none_or_zero;
> +	int shared;
> +	int unmapped;
> +	enum scan_result result;
> +	unsigned int max_ptes_none;
> +	unsigned int max_ptes_swap;
> +	unsigned int max_ptes_shared;
> +};
> +
>  /**
>   * struct khugepaged_scan - cursor for scanning
>   * @mm_head: the head of the mm list to scan
> @@ -696,74 +716,131 @@ static void count_collapse_event(unsigned int order, enum vm_event_item vm_event
>  	count_mthp_stat(order, mthp_event);
>  }
>  
> +/*
> + * pte_check_fail() - A simple helper to set the pte_check_context result and
> + * return PTE_CHECK_FAIL.
> + */
> +static enum pte_check_result pte_check_fail(struct pte_check_context *ctx,
> +		enum scan_result result)
> +{
> +	ctx->result = result;
> +	return PTE_CHECK_FAIL;
> +}
> +
> +/*
> + * collapse_check_pte() - Check if a PTE is suitable for collapse
> + *
> + * Check if a PTE is suitable for collapse based on the following criteria:
> + * - max_pte_* values are not exceeded
> + * - uffd is not active
> + * - lazyfree properties are not present
> + * - only anonymous pages are present
> + *
> + * a helper struct pte_check_context is used to pass and store relevant
> + * information between the collapse_check_pte() function and the caller.
> + *
> + * Return: PTE_CHECK_SUCCEED if the PTE is suitable for collapse,
> + *         PTE_CHECK_FAIL if the PTE is not suitable for collapse,
> + *         PTE_CHECK_CONTINUE if the scan should continue to check the next PTE.

I was thinking if we can make decisions based on scan_result instead of
a new pte_check_result, but pte_check_context->unmapped also changes the
result of collapse_check_pte(). And we will need a new SCAN_CONTINUE to
match PTE_CHECK_CONTINUE.

The changes look good to me.

Acked-by: Zi Yan <ziy@nvidia.com>


-- 
Best Regards,
Yan, Zi