Re: [PATCH v3 01/13] mm/huge_memory: simplify vma_is_specal_huge()

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Mar 20, 2026 at 11:07 AM Lorenzo Stoakes (Oracle)
<ljs@kernel.org> wrote:
>
> This function is confused - it overloads the term 'special' yet again,
> checks for DAX but in many cases the code explicitly excludes DAX before
> invoking the predicate.
>
> It also unnecessarily checks for vma->vm_file - this has to be present for
> a driver to have set VMA_MIXEDMAP_BIT or VMA_PFNMAP_BIT.
>
> In fact, a far simpler form of this is to reverse the DAX predicate and
> return false if DAX is set.
>
> This makes sense from the point of view of 'special' as in
> vm_normal_page(), as DAX actually does potentially have retrievable
> folios.
>
> Also there's no need to have this in mm.h so move it to huge_memory.c.
>
> No functional change intended.
>
> Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

> ---
>  include/linux/huge_mm.h |  4 ++--
>  include/linux/mm.h      | 16 ----------------
>  mm/huge_memory.c        | 30 +++++++++++++++++++++++-------
>  3 files changed, 25 insertions(+), 25 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 8d801ed378db..af726f0aa30d 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -83,7 +83,7 @@ extern struct kobj_attribute thpsize_shmem_enabled_attr;
>   * file is never split and the MAX_PAGECACHE_ORDER limit does not apply to
>   * it.  Same to PFNMAPs where there's neither page* nor pagecache.
>   */
> -#define THP_ORDERS_ALL_SPECIAL         \
> +#define THP_ORDERS_ALL_SPECIAL_DAX     \
>         (BIT(PMD_ORDER) | BIT(PUD_ORDER))
>  #define THP_ORDERS_ALL_FILE_DEFAULT    \
>         ((BIT(MAX_PAGECACHE_ORDER + 1) - 1) & ~BIT(0))
> @@ -92,7 +92,7 @@ extern struct kobj_attribute thpsize_shmem_enabled_attr;
>   * Mask of all large folio orders supported for THP.
>   */
>  #define THP_ORDERS_ALL \
> -       (THP_ORDERS_ALL_ANON | THP_ORDERS_ALL_SPECIAL | THP_ORDERS_ALL_FILE_DEFAULT)
> +       (THP_ORDERS_ALL_ANON | THP_ORDERS_ALL_SPECIAL_DAX | THP_ORDERS_ALL_FILE_DEFAULT)
>
>  enum tva_type {
>         TVA_SMAPS,              /* Exposing "THPeligible:" in smaps. */
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 8aadf115278e..6b07ee99b38b 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -5078,22 +5078,6 @@ long copy_folio_from_user(struct folio *dst_folio,
>                            const void __user *usr_src,
>                            bool allow_pagefault);
>
> -/**
> - * vma_is_special_huge - Are transhuge page-table entries considered special?
> - * @vma: Pointer to the struct vm_area_struct to consider
> - *
> - * Whether transhuge page-table entries are considered "special" following
> - * the definition in vm_normal_page().
> - *
> - * Return: true if transhuge page-table entries should be considered special,
> - * false otherwise.
> - */
> -static inline bool vma_is_special_huge(const struct vm_area_struct *vma)
> -{
> -       return vma_is_dax(vma) || (vma->vm_file &&
> -                                  (vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP)));
> -}
> -
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE || CONFIG_HUGETLBFS */
>
>  #if MAX_NUMNODES > 1
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index e90d08db219d..2775309b317a 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -103,6 +103,14 @@ static inline bool file_thp_enabled(struct vm_area_struct *vma)
>         return !inode_is_open_for_write(inode) && S_ISREG(inode->i_mode);
>  }
>
> +/* If returns true, we are unable to access the VMA's folios. */
> +static bool vma_is_special_huge(const struct vm_area_struct *vma)
> +{
> +       if (vma_is_dax(vma))
> +               return false;
> +       return vma_test_any(vma, VMA_PFNMAP_BIT, VMA_MIXEDMAP_BIT);
> +}
> +
>  unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>                                          vm_flags_t vm_flags,
>                                          enum tva_type type,
> @@ -116,8 +124,8 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>         /* Check the intersection of requested and supported orders. */
>         if (vma_is_anonymous(vma))
>                 supported_orders = THP_ORDERS_ALL_ANON;
> -       else if (vma_is_special_huge(vma))
> -               supported_orders = THP_ORDERS_ALL_SPECIAL;
> +       else if (vma_is_dax(vma) || vma_is_special_huge(vma))
> +               supported_orders = THP_ORDERS_ALL_SPECIAL_DAX;
>         else
>                 supported_orders = THP_ORDERS_ALL_FILE_DEFAULT;
>
> @@ -2338,7 +2346,7 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>                                                 tlb->fullmm);
>         arch_check_zapped_pmd(vma, orig_pmd);
>         tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
> -       if (!vma_is_dax(vma) && vma_is_special_huge(vma)) {
> +       if (vma_is_special_huge(vma)) {
>                 if (arch_needs_pgtable_deposit())
>                         zap_deposited_table(tlb->mm, pmd);
>                 spin_unlock(ptl);
> @@ -2840,7 +2848,7 @@ int zap_huge_pud(struct mmu_gather *tlb, struct vm_area_struct *vma,
>         orig_pud = pudp_huge_get_and_clear_full(vma, addr, pud, tlb->fullmm);
>         arch_check_zapped_pud(vma, orig_pud);
>         tlb_remove_pud_tlb_entry(tlb, pud, addr);
> -       if (!vma_is_dax(vma) && vma_is_special_huge(vma)) {
> +       if (vma_is_special_huge(vma)) {
>                 spin_unlock(ptl);
>                 /* No zero page support yet */
>         } else {
> @@ -2991,7 +2999,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>                  */
>                 if (arch_needs_pgtable_deposit())
>                         zap_deposited_table(mm, pmd);
> -               if (!vma_is_dax(vma) && vma_is_special_huge(vma))
> +               if (vma_is_special_huge(vma))
>                         return;
>                 if (unlikely(pmd_is_migration_entry(old_pmd))) {
>                         const softleaf_t old_entry = softleaf_from_pmd(old_pmd);
> @@ -4517,8 +4525,16 @@ static void split_huge_pages_all(void)
>
>  static inline bool vma_not_suitable_for_thp_split(struct vm_area_struct *vma)
>  {
> -       return vma_is_special_huge(vma) || (vma->vm_flags & VM_IO) ||
> -                   is_vm_hugetlb_page(vma);
> +       if (vma_is_dax(vma))
> +               return true;
> +       if (vma_is_special_huge(vma))
> +               return true;
> +       if (vma_test(vma, VMA_IO_BIT))
> +               return true;
> +       if (is_vm_hugetlb_page(vma))
> +               return true;
> +
> +       return false;
>  }
>
>  static int split_huge_pages_pid(int pid, unsigned long vaddr_start,
> --
> 2.53.0
>