Re: [PATCH] mm/memory: remove unused vmf_insert_mixed_mkwrite()

["Jose A. Perez de Azpillaga" <azpijr@gmail.com>]

On Tue, Sep 29, 2026 at 11:37:06PM +0900, Sang-Heon Jeon wrote:
> Since commit 38607c62b34b ("fs/dax: properly refcount fs dax pages"),
> vmf_insert_mixed_mkwrite() has no callers, so the mkwrite argument of
> __vm_insert_mixed() and insert_pfn() is always false.
>
> So remove the function, the mkwrite argument and the unreachable
> mkwrite branches. Also merge __vm_insert_mixed() into vmf_insert_mixed().
>
> No functional change.
>
> Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
> ---
>  include/linux/mm.h |  2 --
>  mm/memory.c        | 76 ++++++++++++----------------------------------
>  2 files changed, 19 insertions(+), 59 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index c038d06825c3..2ac734efd4ec 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -5008,8 +5008,6 @@ vm_fault_t vmf_insert_pfn_prot(struct vm_area_struct *vma, unsigned long addr,
>  			unsigned long pfn, pgprot_t pgprot);
>  vm_fault_t vmf_insert_mixed(struct vm_area_struct *vma, unsigned long addr,
>  			unsigned long pfn);
> -vm_fault_t vmf_insert_mixed_mkwrite(struct vm_area_struct *vma,
> -		unsigned long addr, unsigned long pfn);
>  int vm_iomap_memory(struct vm_area_struct *vma, phys_addr_t start, unsigned long len);
>
>  static inline vm_fault_t vmf_insert_page(struct vm_area_struct *vma,
> diff --git a/mm/memory.c b/mm/memory.c
> index 330cde31bf8b..1f83a26f8733 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2453,7 +2453,14 @@ static int insert_page_into_pte_locked(struct vm_area_struct *vma, pte_t *pte,
>  		if (!mkwrite)
>  			return -EBUSY;
>
> -		/* see insert_pfn(). */
> +		/*
> +		 * For read faults on private mappings the PFN passed in may
> +		 * not match the PFN we have mapped if the mapped PFN is a
> +		 * writeable COW page.  In the mkwrite case we are creating a
> +		 * writable PTE for a shared mapping and we expect the PFNs to
> +		 * match. If they don't match, we are likely racing with block
> +		 * allocation and mapping invalidation.
> +		 */
>  		if (pte_pfn(pteval) != page_to_pfn(page)) {
>  			WARN_ON_ONCE(!is_zero_pfn(pte_pfn(pteval)));
>  			return -EFAULT;
> @@ -2858,7 +2865,7 @@ int vm_map_pages_zero(struct vm_area_struct *vma, struct page **pages,
>  EXPORT_SYMBOL(vm_map_pages_zero);
>
>  static vm_fault_t insert_pfn(struct vm_area_struct *vma, unsigned long addr,
> -			unsigned long pfn, pgprot_t prot, bool mkwrite)
> +			unsigned long pfn, pgprot_t prot)
>  {
>  	struct mm_struct *mm = vma->vm_mm;
>  	pte_t *pte, entry;
> @@ -2868,38 +2875,12 @@ static vm_fault_t insert_pfn(struct vm_area_struct *vma, unsigned long addr,
>  	if (!pte)
>  		return VM_FAULT_OOM;
>  	entry = ptep_get(pte);
> -	if (!pte_none(entry)) {
> -		if (mkwrite) {
> -			/*
> -			 * For read faults on private mappings the PFN passed
> -			 * in may not match the PFN we have mapped if the
> -			 * mapped PFN is a writeable COW page.  In the mkwrite
> -			 * case we are creating a writable PTE for a shared
> -			 * mapping and we expect the PFNs to match. If they
> -			 * don't match, we are likely racing with block
> -			 * allocation and mapping invalidation so just skip the
> -			 * update.
> -			 */
> -			if (pte_pfn(entry) != pfn) {
> -				WARN_ON_ONCE(!is_zero_pfn(pte_pfn(entry)));
> -				goto out_unlock;
> -			}
> -			entry = pte_mkyoung(entry);
> -			entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> -			if (ptep_set_access_flags(vma, addr, pte, entry, 1))
> -				update_mmu_cache(vma, addr, pte);
> -		}
> +	if (!pte_none(entry))
>  		goto out_unlock;
> -	}
>
>  	/* Ok, finally just insert the thing.. */
>  	entry = pte_mkspecial(pfn_pte(pfn, prot));
>
> -	if (mkwrite) {
> -		entry = pte_mkyoung(entry);
> -		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> -	}
> -
>  	set_pte_at(mm, addr, pte, entry);
>  	update_mmu_cache(vma, addr, pte); /* XXX: why not for insert_page? */
>
> @@ -2967,7 +2948,7 @@ vm_fault_t vmf_insert_pfn_prot(struct vm_area_struct *vma, unsigned long addr,
>
>  	pfnmap_setup_cachemode_pfn(pfn, &pgprot);
>
> -	return insert_pfn(vma, addr, pfn, pgprot, false);
> +	return insert_pfn(vma, addr, pfn, pgprot);
>  }
>  EXPORT_SYMBOL(vmf_insert_pfn_prot);
>
> @@ -2998,11 +2979,9 @@ vm_fault_t vmf_insert_pfn(struct vm_area_struct *vma, unsigned long addr,
>  }
>  EXPORT_SYMBOL(vmf_insert_pfn);
>
> -static bool vm_mixed_ok(struct vm_area_struct *vma, unsigned long pfn,
> -			bool mkwrite)
> +static bool vm_mixed_ok(struct vm_area_struct *vma, unsigned long pfn)
>  {
> -	if (unlikely(is_zero_pfn(pfn)) &&
> -	    (mkwrite || !vm_mixed_zeropage_allowed(vma)))
> +	if (unlikely(is_zero_pfn(pfn)) && !vm_mixed_zeropage_allowed(vma))
>  		return false;
>  	/* these checks mirror the abort conditions in vm_normal_page */
>  	if (vma->vm_flags & VM_MIXEDMAP)
> @@ -3012,13 +2991,13 @@ static bool vm_mixed_ok(struct vm_area_struct *vma, unsigned long pfn,
>  	return false;
>  }
>
> -static vm_fault_t __vm_insert_mixed(struct vm_area_struct *vma,
> -		unsigned long addr, unsigned long pfn, bool mkwrite)
> +vm_fault_t vmf_insert_mixed(struct vm_area_struct *vma, unsigned long addr,
> +		unsigned long pfn)
>  {
>  	pgprot_t pgprot = vma->vm_page_prot;
>  	int err;
>
> -	if (!vm_mixed_ok(vma, pfn, mkwrite))
> +	if (!vm_mixed_ok(vma, pfn))
>  		return VM_FAULT_SIGBUS;
>
>  	if (addr < vma->vm_start || addr >= vma->vm_end)
> @@ -3045,9 +3024,9 @@ static vm_fault_t __vm_insert_mixed(struct vm_area_struct *vma,
>  		 * result in pfn_t_has_page() == false.
>  		 */
>  		page = pfn_to_page(pfn);
> -		err = insert_page(vma, addr, page, pgprot, mkwrite);
> +		err = insert_page(vma, addr, page, pgprot, false);
>  	} else {
> -		return insert_pfn(vma, addr, pfn, pgprot, mkwrite);
> +		return insert_pfn(vma, addr, pfn, pgprot);
>  	}
>
>  	if (err == -ENOMEM)
> @@ -3057,6 +3036,7 @@ static vm_fault_t __vm_insert_mixed(struct vm_area_struct *vma,
>
>  	return VM_FAULT_NOPAGE;
>  }
> +EXPORT_SYMBOL(vmf_insert_mixed);
>
>  vm_fault_t vmf_insert_page_mkwrite(struct vm_fault *vmf, struct page *page,
>  			bool write)
> @@ -3078,24 +3058,6 @@ vm_fault_t vmf_insert_page_mkwrite(struct vm_fault *vmf, struct page *page,
>  }
>  EXPORT_SYMBOL_GPL(vmf_insert_page_mkwrite);
>
> -vm_fault_t vmf_insert_mixed(struct vm_area_struct *vma, unsigned long addr,
> -		unsigned long pfn)
> -{
> -	return __vm_insert_mixed(vma, addr, pfn, false);
> -}
> -EXPORT_SYMBOL(vmf_insert_mixed);
> -
> -/*
> - *  If the insertion of PTE failed because someone else already added a
> - *  different entry in the mean time, we treat that as success as we assume
> - *  the same entry was actually inserted.
> - */

it reads to me like it describes the -EBUSY handling rather than
vmf_insert_mixed_mkwrite() itself, and that handling is still there in
__vm_insert_mixed(), which this patch renames to vmf_insert_mixed(). so
I would have expected it to move above the "if (err < 0 && err != -EBUSY)"
there rather than go away with the wrapper.

am I missing a reason to drop it? because, if not, moving it would keep
one explanation we have for why -EBUSY is not treated as an error there.

> -vm_fault_t vmf_insert_mixed_mkwrite(struct vm_area_struct *vma,
> -		unsigned long addr, unsigned long pfn)
> -{
> -	return __vm_insert_mixed(vma, addr, pfn, true);
> -}
> -
>  /*
>   * maps a range of physical memory into the requested pages. the old
>   * mappings are removed. any references to nonexistent pages results
>
> base-commit: 1c2b8d2725f84b43fabe3b3e9628c91db8ca6c65
> --
> 2.43.0
>
>

overall, looks good to me. but I want to know about the comment.

--
cheers,
jose a. p-a