Re: [PATCH v4 15/25] huge_memory: Add vmf_insert_folio_pud()

[David Hildenbrand <david@redhat.com>]

On 17.12.24 06:12, Alistair Popple wrote:
> Currently DAX folio/page reference counts are managed differently to
> normal pages. To allow these to be managed the same as normal pages
> introduce vmf_insert_folio_pud. This will map the entire PUD-sized folio
> and take references as it would for a normally mapped page.
> 
> This is distinct from the current mechanism, vmf_insert_pfn_pud, which
> simply inserts a special devmap PUD entry into the page table without
> holding a reference to the page for the mapping.
> 
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> ---
>   include/linux/huge_mm.h | 11 +++++-
>   mm/huge_memory.c        | 96 ++++++++++++++++++++++++++++++++++++------
>   2 files changed, 95 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 93e509b..012137b 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -39,6 +39,7 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>   
>   vm_fault_t vmf_insert_pfn_pmd(struct vm_fault *vmf, pfn_t pfn, bool write);
>   vm_fault_t vmf_insert_pfn_pud(struct vm_fault *vmf, pfn_t pfn, bool write);
> +vm_fault_t vmf_insert_folio_pud(struct vm_fault *vmf, struct folio *folio, bool write);
>   
>   enum transparent_hugepage_flag {
>   	TRANSPARENT_HUGEPAGE_UNSUPPORTED,
> @@ -458,6 +459,11 @@ static inline bool is_huge_zero_pmd(pmd_t pmd)
>   	return pmd_present(pmd) && READ_ONCE(huge_zero_pfn) == pmd_pfn(pmd);
>   }
>   
> +static inline bool is_huge_zero_pud(pud_t pud)
> +{
> +	return false;
> +}
> +
>   struct folio *mm_get_huge_zero_folio(struct mm_struct *mm);
>   void mm_put_huge_zero_folio(struct mm_struct *mm);
>   
> @@ -604,6 +610,11 @@ static inline bool is_huge_zero_pmd(pmd_t pmd)
>   	return false;
>   }
>   
> +static inline bool is_huge_zero_pud(pud_t pud)
> +{
> +	return false;
> +}
> +

I'm really not a fan of these, because I assume we will never ever 
implement these any time soon. (who will waste 1 GiG or more on faster 
reading of 0s?)


>   static inline void mm_put_huge_zero_folio(struct mm_struct *mm)
>   {
>   	return;
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 120cd2c..5081808 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1482,19 +1482,17 @@ static void insert_pfn_pud(struct vm_area_struct *vma, unsigned long addr,
>   	struct mm_struct *mm = vma->vm_mm;
>   	pgprot_t prot = vma->vm_page_prot;
>   	pud_t entry;
> -	spinlock_t *ptl;
>   
> -	ptl = pud_lock(mm, pud);
>   	if (!pud_none(*pud)) {
>   		if (write) {
>   			if (WARN_ON_ONCE(pud_pfn(*pud) != pfn_t_to_pfn(pfn)))
> -				goto out_unlock;
> +				return;
>   			entry = pud_mkyoung(*pud);
>   			entry = maybe_pud_mkwrite(pud_mkdirty(entry), vma);
>   			if (pudp_set_access_flags(vma, addr, pud, entry, 1))
>   				update_mmu_cache_pud(vma, addr, pud);
>   		}
> -		goto out_unlock;
> +		return;
>   	}
>   
>   	entry = pud_mkhuge(pfn_t_pud(pfn, prot));
> @@ -1508,9 +1506,6 @@ static void insert_pfn_pud(struct vm_area_struct *vma, unsigned long addr,
>   	}
>   	set_pud_at(mm, addr, pud, entry);
>   	update_mmu_cache_pud(vma, addr, pud);
> -
> -out_unlock:
> -	spin_unlock(ptl);
>   }
>   
>   /**
> @@ -1528,6 +1523,7 @@ vm_fault_t vmf_insert_pfn_pud(struct vm_fault *vmf, pfn_t pfn, bool write)
>   	unsigned long addr = vmf->address & PUD_MASK;
>   	struct vm_area_struct *vma = vmf->vma;
>   	pgprot_t pgprot = vma->vm_page_prot;
> +	spinlock_t *ptl;
>   
>   	/*
>   	 * If we had pud_special, we could avoid all these restrictions,
> @@ -1545,10 +1541,55 @@ vm_fault_t vmf_insert_pfn_pud(struct vm_fault *vmf, pfn_t pfn, bool write)
>   
>   	track_pfn_insert(vma, &pgprot, pfn);
>   
> +	ptl = pud_lock(vma->vm_mm, vmf->pud);
>   	insert_pfn_pud(vma, addr, vmf->pud, pfn, write);
> +	spin_unlock(ptl);
> +
>   	return VM_FAULT_NOPAGE;
>   }
>   EXPORT_SYMBOL_GPL(vmf_insert_pfn_pud);
> +
> +/**
> + * vmf_insert_folio_pud - insert a pud size folio mapped by a pud entry
> + * @vmf: Structure describing the fault
> + * @pfn: pfn of the page to insert
> + * @write: whether it's a write fault
> + *
> + * Return: vm_fault_t value.
> + */
> +vm_fault_t vmf_insert_folio_pud(struct vm_fault *vmf, struct folio *folio, bool write)
> +{
> +	struct vm_area_struct *vma = vmf->vma;
> +	unsigned long addr = vmf->address & PUD_MASK;
> +	pfn_t pfn = pfn_to_pfn_t(folio_pfn(folio));
> +	pud_t *pud = vmf->pud;
> +	pgprot_t prot = vma->vm_page_prot;

See below, pfn, prot and page can likely go.

> +	struct mm_struct *mm = vma->vm_mm;
> +	spinlock_t *ptl;
> +	struct page *page;
> +
> +	if (addr < vma->vm_start || addr >= vma->vm_end)
> +		return VM_FAULT_SIGBUS;
> +
> +	if (WARN_ON_ONCE(folio_order(folio) != PUD_ORDER))
> +		return VM_FAULT_SIGBUS;
> +
> +	track_pfn_insert(vma, &prot, pfn);

Oh, why is that required? We are inserting a folio and start messing 
with VM_PAT on x86 that only applies to VM_PFNMAP mappings? :)

> +
> +	ptl = pud_lock(mm, pud);
> +	if (pud_none(*vmf->pud)) {
> +		page = pfn_t_to_page(pfn);

Why are we suddenly working with that pfn_t whichcraft? :)


> +		folio = page_folio(page);

Ehm, you got the folio ... passed into this function?

Why can't that simply be

folio_get(folio);
folio_add_file_rmap_pud(folio, folio_page(folio, 0), vma);

> +		folio_get(folio);
> +		folio_add_file_rmap_pud(folio, page, vma);
> +		add_mm_counter(mm, mm_counter_file(folio), HPAGE_PUD_NR);
> +	}
> +	insert_pfn_pud(vma, addr, vmf->pud, pfn, write);
> +	spin_unlock(ptl);
> +
> +	return VM_FAULT_NOPAGE;
> +}
> +EXPORT_SYMBOL_GPL(vmf_insert_folio_pud);
>   #endif /* CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD */
>   
>   void touch_pmd(struct vm_area_struct *vma, unsigned long addr,
> @@ -2146,7 +2187,8 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>   			zap_deposited_table(tlb->mm, pmd);
>   		spin_unlock(ptl);
>   	} else if (is_huge_zero_pmd(orig_pmd)) {
> -		zap_deposited_table(tlb->mm, pmd);
> +		if (!vma_is_dax(vma) || arch_needs_pgtable_deposit())
> +			zap_deposited_table(tlb->mm, pmd);
>   		spin_unlock(ptl);
>   	} else {
>   		struct folio *folio = NULL;
> @@ -2634,12 +2676,24 @@ 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_special_huge(vma)) {
> +	if (!vma_is_dax(vma) && vma_is_special_huge(vma)) {
>   		spin_unlock(ptl);
>   		/* No zero page support yet */
>   	} else {
> -		/* No support for anonymous PUD pages yet */
> -		BUG();
> +		struct page *page = NULL;
> +		struct folio *folio;
> +
> +		/* No support for anonymous PUD pages or migration yet */
> +		BUG_ON(vma_is_anonymous(vma) || !pud_present(orig_pud));

VM_WARN_ON_ONCE().

> +
> +		page = pud_page(orig_pud);
> +		folio = page_folio(page);
> +		folio_remove_rmap_pud(folio, page, vma);
> +		VM_BUG_ON_PAGE(!PageHead(page), page);

Please drop that or so something like

VM_WARN_ON_ONCE(page != folio_page(folio, 0));

> +		add_mm_counter(tlb->mm, mm_counter_file(folio), -HPAGE_PUD_NR);
> +
> +		spin_unlock(ptl);
> +		tlb_remove_page_size(tlb, page, HPAGE_PUD_SIZE);
>   	}
>   	return 1;
>   }
> @@ -2647,6 +2701,8 @@ int zap_huge_pud(struct mmu_gather *tlb, struct vm_area_struct *vma,
>   static void __split_huge_pud_locked(struct vm_area_struct *vma, pud_t *pud,
>   		unsigned long haddr)
>   {
> +	pud_t old_pud;
> +
>   	VM_BUG_ON(haddr & ~HPAGE_PUD_MASK);
>   	VM_BUG_ON_VMA(vma->vm_start > haddr, vma);
>   	VM_BUG_ON_VMA(vma->vm_end < haddr + HPAGE_PUD_SIZE, vma);
> @@ -2654,7 +2710,23 @@ static void __split_huge_pud_locked(struct vm_area_struct *vma, pud_t *pud,
>   
>   	count_vm_event(THP_SPLIT_PUD);
>   
> -	pudp_huge_clear_flush(vma, haddr, pud);
> +	old_pud = pudp_huge_clear_flush(vma, haddr, pud);
> +	if (is_huge_zero_pud(old_pud))
> +		return;
> +
> +	if (vma_is_dax(vma)) {

Maybe you want

if (!vma_is_dax(vma))
	return;

To then reduce alignment. I suspect all other splitting code (besides 
anon memory) will want to do the same thing here in the future.

-- 
Cheers,

David / dhildenb