Re: [PATCH 1/8] mm/huge_memory: simplify vma_is_specal_huge()

[Lorenzo Stoakes (Oracle)]

On Thu, Mar 19, 2026 at 11:16:20AM +0800, Qi Zheng wrote:
>
>
> On 3/19/26 4:39 AM, Lorenzo Stoakes (Oracle) 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.
> >
> > Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@xxxxxxxxxx>
> > ---
> >   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 bd7f0e1d8094..61fda1672b29 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	\
>
> As mentioned in the comments, the pfnmap case is also include in the
> 'special' case, right?

Yeah special = pfnmap, mixedmap. so renaming to SPECIAL_DAX to make clear it's
either dax or 'special' in the meaning of vm_normal_page().

>
> >   	(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 6f0a3edb24e1..50d68b092204 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -5077,22 +5077,6 @@ 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 3fc02913b63e..f76edfa91e96 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -100,6 +100,14 @@ 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(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,
> > @@ -113,8 +121,8 @@ 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;
> > @@ -2431,7 +2439,7 @@ 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);
> > @@ -2933,7 +2941,7 @@ 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 {
> > @@ -3084,7 +3092,7 @@ 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);
> > @@ -4645,8 +4653,16 @@ 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,
>