[PATCH 05/12] mm/memory: Add dax_insert_pfn

[Alistair Popple <apopple@nvidia.com>]

Currently to map a DAX page the DAX driver calls vmf_insert_pfn. This
creates a special devmap PTE entry for the pfn but does not take a
reference on the underlying struct page for the mapping. This is
because DAX page refcounts are treated specially, as indicated by the
presence of a devmap entry.

To allow DAX page refcounts to be managed the same as normal page
refcounts introduce dax_insert_pfn. This will take a reference on the
underlying page much the same as vmf_insert_page, except it also
permits upgrading an existing mapping to be writable if
requested/possible.

Signed-off-by: Alistair Popple <apopple@nvidia.com>

---

Updates from v1:

 - Re-arrange code in insert_page_into_pte_locked() based on comments
   from Jan Kara.

 - Call mkdrity/mkyoung for the mkwrite case, also suggested by Jan.
---
 include/linux/mm.h |  1 +-
 mm/memory.c        | 83 ++++++++++++++++++++++++++++++++++++++++++-----
 2 files changed, 76 insertions(+), 8 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index b0ff06d..ae6d713 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3463,6 +3463,7 @@ int vm_map_pages(struct vm_area_struct *vma, struct page **pages,
 				unsigned long num);
 int vm_map_pages_zero(struct vm_area_struct *vma, struct page **pages,
 				unsigned long num);
+vm_fault_t dax_insert_pfn(struct vm_fault *vmf, pfn_t pfn_t, bool write);
 vm_fault_t vmf_insert_pfn(struct vm_area_struct *vma, unsigned long addr,
 			unsigned long pfn);
 vm_fault_t vmf_insert_pfn_prot(struct vm_area_struct *vma, unsigned long addr,
diff --git a/mm/memory.c b/mm/memory.c
index d2785fb..368e15d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2039,19 +2039,47 @@ static int validate_page_before_insert(struct vm_area_struct *vma,
 }
 
 static int insert_page_into_pte_locked(struct vm_area_struct *vma, pte_t *pte,
-			unsigned long addr, struct page *page, pgprot_t prot)
+				unsigned long addr, struct page *page,
+				pgprot_t prot, bool mkwrite)
 {
 	struct folio *folio = page_folio(page);
+	pte_t entry = ptep_get(pte);
 	pte_t pteval;
 
-	if (!pte_none(ptep_get(pte)))
-		return -EBUSY;
+	if (!pte_none(entry)) {
+		if (!mkwrite)
+			return -EBUSY;
+
+		/*
+		 * 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) != page_to_pfn(page)) {
+			WARN_ON_ONCE(!is_zero_pfn(pte_pfn(entry)));
+			return -EFAULT;
+		}
+		entry = maybe_mkwrite(entry, vma);
+		entry = pte_mkyoung(entry);
+		if (ptep_set_access_flags(vma, addr, pte, entry, 1))
+			update_mmu_cache(vma, addr, pte);
+		return 0;
+	}
+
 	/* Ok, finally just insert the thing.. */
 	pteval = mk_pte(page, prot);
 	if (unlikely(is_zero_folio(folio))) {
 		pteval = pte_mkspecial(pteval);
 	} else {
 		folio_get(folio);
+		entry = mk_pte(page, prot);
+		if (mkwrite) {
+			entry = pte_mkyoung(entry);
+			entry = maybe_mkwrite(pte_mkdirty(entry), vma);
+		}
 		inc_mm_counter(vma->vm_mm, mm_counter_file(folio));
 		folio_add_file_rmap_pte(folio, page, vma);
 	}
@@ -2060,7 +2088,7 @@ static int insert_page_into_pte_locked(struct vm_area_struct *vma, pte_t *pte,
 }
 
 static int insert_page(struct vm_area_struct *vma, unsigned long addr,
-			struct page *page, pgprot_t prot)
+			struct page *page, pgprot_t prot, bool mkwrite)
 {
 	int retval;
 	pte_t *pte;
@@ -2073,7 +2101,8 @@ static int insert_page(struct vm_area_struct *vma, unsigned long addr,
 	pte = get_locked_pte(vma->vm_mm, addr, &ptl);
 	if (!pte)
 		goto out;
-	retval = insert_page_into_pte_locked(vma, pte, addr, page, prot);
+	retval = insert_page_into_pte_locked(vma, pte, addr, page, prot,
+					mkwrite);
 	pte_unmap_unlock(pte, ptl);
 out:
 	return retval;
@@ -2087,7 +2116,7 @@ static int insert_page_in_batch_locked(struct vm_area_struct *vma, pte_t *pte,
 	err = validate_page_before_insert(vma, page);
 	if (err)
 		return err;
-	return insert_page_into_pte_locked(vma, pte, addr, page, prot);
+	return insert_page_into_pte_locked(vma, pte, addr, page, prot, false);
 }
 
 /* insert_pages() amortizes the cost of spinlock operations
@@ -2223,7 +2252,7 @@ int vm_insert_page(struct vm_area_struct *vma, unsigned long addr,
 		BUG_ON(vma->vm_flags & VM_PFNMAP);
 		vm_flags_set(vma, VM_MIXEDMAP);
 	}
-	return insert_page(vma, addr, page, vma->vm_page_prot);
+	return insert_page(vma, addr, page, vma->vm_page_prot, false);
 }
 EXPORT_SYMBOL(vm_insert_page);
 
@@ -2503,7 +2532,7 @@ static vm_fault_t __vm_insert_mixed(struct vm_area_struct *vma,
 		 * result in pfn_t_has_page() == false.
 		 */
 		page = pfn_to_page(pfn_t_to_pfn(pfn));
-		err = insert_page(vma, addr, page, pgprot);
+		err = insert_page(vma, addr, page, pgprot, mkwrite);
 	} else {
 		return insert_pfn(vma, addr, pfn, pgprot, mkwrite);
 	}
@@ -2516,6 +2545,44 @@ static vm_fault_t __vm_insert_mixed(struct vm_area_struct *vma,
 	return VM_FAULT_NOPAGE;
 }
 
+vm_fault_t dax_insert_pfn(struct vm_fault *vmf, pfn_t pfn_t, bool write)
+{
+	struct vm_area_struct *vma = vmf->vma;
+	pgprot_t pgprot = vma->vm_page_prot;
+	unsigned long pfn = pfn_t_to_pfn(pfn_t);
+	struct page *page = pfn_to_page(pfn);
+	unsigned long addr = vmf->address;
+	int err;
+
+	if (addr < vma->vm_start || addr >= vma->vm_end)
+		return VM_FAULT_SIGBUS;
+
+	track_pfn_insert(vma, &pgprot, pfn_t);
+
+	if (!pfn_modify_allowed(pfn, pgprot))
+		return VM_FAULT_SIGBUS;
+
+	/*
+	 * We refcount the page normally so make sure pfn_valid is true.
+	 */
+	if (!pfn_t_valid(pfn_t))
+		return VM_FAULT_SIGBUS;
+
+	WARN_ON_ONCE(pfn_t_devmap(pfn_t));
+
+	if (WARN_ON(is_zero_pfn(pfn) && write))
+		return VM_FAULT_SIGBUS;
+
+	err = insert_page(vma, addr, page, pgprot, write);
+	if (err == -ENOMEM)
+		return VM_FAULT_OOM;
+	if (err < 0 && err != -EBUSY)
+		return VM_FAULT_SIGBUS;
+
+	return VM_FAULT_NOPAGE;
+}
+EXPORT_SYMBOL_GPL(dax_insert_pfn);
+
 vm_fault_t vmf_insert_mixed(struct vm_area_struct *vma, unsigned long addr,
 		pfn_t pfn)
 {
-- 
git-series 0.9.1