Re: [PATCH mm-unstable v1 1/5] mm: consolidate anonymous folio PTE mapping into helpers

[Joshua Hahn <joshua.hahnjy@gmail.com>]

On Wed, 11 Feb 2026 19:18:31 -0700 Nico Pache <npache@redhat.com> wrote:

Hello Nico,

Thank you for the patch! I hope you are having a good day.

> The anonymous page fault handler in do_anonymous_page() open-codes the
> sequence to map a newly allocated anonymous folio at the PTE level:
> 	- construct the PTE entry
> 	- add rmap
> 	- add to LRU
> 	- set the PTEs
> 	- update the MMU cache.
> 
> Introduce a two helpers to consolidate this duplicated logic, mirroring the
> existing map_anon_folio_pmd_nopf() pattern for PMD-level mappings:
> 
> 	map_anon_folio_pte_nopf(): constructs the PTE entry, takes folio
> 	references, adds anon rmap and LRU. This function also handles the
> 	uffd_wp that can occur in the pf variant.
> 
> 	map_anon_folio_pte_pf(): extends the nopf variant to handle MM_ANONPAGES
> 	counter updates, and mTHP fault allocation statistics for the page fault
> 	path.
> 
> The zero-page read path in do_anonymous_page() is also untangled from the
> shared setpte label, since it does not allocate a folio and should not
> share the same mapping sequence as the write path. Make nr_pages = 1
> rather than relying on the variable. This makes it more clear that we
> are operating on the zero page only.
> 
> This refactoring will also help reduce code duplication between mm/memory.c
> and mm/khugepaged.c, and provides a clean API for PTE-level anonymous folio
> mapping that can be reused by future callers.

It seems that based on this description, there should be no functional change
in the code below. Is that correct?

> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>  include/linux/mm.h |  4 ++++
>  mm/memory.c        | 56 ++++++++++++++++++++++++++++++----------------
>  2 files changed, 41 insertions(+), 19 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index f8a8fd47399c..c3aa1f51e020 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4916,4 +4916,8 @@ static inline bool snapshot_page_is_faithful(const struct page_snapshot *ps)
>  
>  void snapshot_page(struct page_snapshot *ps, const struct page *page);
>  
> +void map_anon_folio_pte_nopf(struct folio *folio, pte_t *pte,
> +		struct vm_area_struct *vma, unsigned long addr,
> +		bool uffd_wp);
> +
>  #endif /* _LINUX_MM_H */
> diff --git a/mm/memory.c b/mm/memory.c
> index 8c19af97f0a0..61c2277c9d9f 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5211,6 +5211,35 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
>  	return folio_prealloc(vma->vm_mm, vma, vmf->address, true);
>  }
>  
> +

^^^ extra newline?

> +void map_anon_folio_pte_nopf(struct folio *folio, pte_t *pte,
> +		struct vm_area_struct *vma, unsigned long addr,
> +		bool uffd_wp)
> +{
> +	pte_t entry = folio_mk_pte(folio, vma->vm_page_prot);
> +	unsigned int nr_pages = folio_nr_pages(folio);

Just reading through the code below on what was deleted and what was added,
Maybe we are missing a pte_sw_mkyoung(entry) here? Seems like this would
matter for MIPS systmes, but I couldn't find this change in the changelog.

> +	entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> +	if (uffd_wp)
> +		entry = pte_mkuffd_wp(entry);

The ordering here was also changed, but it wasn't immediately obvious to me
why it was changed.

> +
> +	folio_ref_add(folio, nr_pages - 1);
> +	folio_add_new_anon_rmap(folio, vma, addr, RMAP_EXCLUSIVE);
> +	folio_add_lru_vma(folio, vma);
> +	set_ptes(vma->vm_mm, addr, pte, entry, nr_pages);
> +	update_mmu_cache_range(NULL, vma, addr, pte, nr_pages);
> +}
> +
> +static void map_anon_folio_pte_pf(struct folio *folio, pte_t *pte,
> +		struct vm_area_struct *vma, unsigned long addr,
> +		unsigned int nr_pages, bool uffd_wp)
> +{
> +	map_anon_folio_pte_nopf(folio, pte, vma, addr, uffd_wp);
> +	add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages);
> +	count_mthp_stat(folio_order(folio), MTHP_STAT_ANON_FAULT_ALLOC);
> +}
> +
> +

^^^ extra newline?

>  /*
>   * We enter with non-exclusive mmap_lock (to exclude vma changes,
>   * but allow concurrent faults), and pte mapped but not yet locked.
> @@ -5257,7 +5286,13 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>  			pte_unmap_unlock(vmf->pte, vmf->ptl);
>  			return handle_userfault(vmf, VM_UFFD_MISSING);
>  		}
> -		goto setpte;
> +		if (vmf_orig_pte_uffd_wp(vmf))
> +			entry = pte_mkuffd_wp(entry);
> +		set_pte_at(vma->vm_mm, addr, vmf->pte, entry);
> +
> +		/* No need to invalidate - it was non-present before */
> +		update_mmu_cache_range(vmf, vma, addr, vmf->pte, /*nr_pages=*/ 1);

NIT: Should we try to keep the line under 80 columns? ; -)

> +		goto unlock;
>  	}
>  
>  	/* Allocate our own private page. */
> @@ -5281,11 +5316,6 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>  	 */
>  	__folio_mark_uptodate(folio);
>  
> -	entry = folio_mk_pte(folio, vma->vm_page_prot);
> -	entry = pte_sw_mkyoung(entry);
> -	if (vma->vm_flags & VM_WRITE)
> -		entry = pte_mkwrite(pte_mkdirty(entry), vma);
> -
>  	vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, addr, &vmf->ptl);
>  	if (!vmf->pte)
>  		goto release;
> @@ -5307,19 +5337,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>  		folio_put(folio);
>  		return handle_userfault(vmf, VM_UFFD_MISSING);
>  	}
> -
> -	folio_ref_add(folio, nr_pages - 1);
> -	add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages);
> -	count_mthp_stat(folio_order(folio), MTHP_STAT_ANON_FAULT_ALLOC);
> -	folio_add_new_anon_rmap(folio, vma, addr, RMAP_EXCLUSIVE);
> -	folio_add_lru_vma(folio, vma);
> -setpte:
> -	if (vmf_orig_pte_uffd_wp(vmf))
> -		entry = pte_mkuffd_wp(entry);
> -	set_ptes(vma->vm_mm, addr, vmf->pte, entry, nr_pages);
> -
> -	/* No need to invalidate - it was non-present before */
> -	update_mmu_cache_range(vmf, vma, addr, vmf->pte, nr_pages);
> +	map_anon_folio_pte_pf(folio, vmf->pte, vma, addr, nr_pages, vmf_orig_pte_uffd_wp(vmf));

NIT: Maybe here as well?

>  unlock:
>  	if (vmf->pte)
>  		pte_unmap_unlock(vmf->pte, vmf->ptl);
> -- 
> 2.53.0

Thank you for the patch again. I hope you have a great day!
Joshua