[PATCH RFC 03/10] mm: Add pud support for _PAGE_SPECIAL

[Joao Martins]

Currently vmf_insert_pfn_pud only works with devmap
and BUG_ON otherwise. Add support for handling
page special when pfn_t has it marked with PFN_SPECIAL.

Usage of this type of pages aren't expected to do GUP
hence return no pages on gup_huge_pud() much like how
it is done for ptes on gup_pte_range() and for pmds on
gup_huge_pmd().

This allows device-dax to handle 1G hugepages without
struct pages.

Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
---
 arch/x86/include/asm/pgtable.h | 18 +++++++++++++++++-
 mm/gup.c                       |  3 +++
 mm/huge_memory.c               |  8 +++++---
 mm/memory.c                    |  3 ++-
 4 files changed, 27 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 60351c0c15fe..2027c063fa16 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -261,7 +261,7 @@ static inline int pmd_trans_huge(pmd_t pmd)
 #ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
 static inline int pud_trans_huge(pud_t pud)
 {
-	return (pud_val(pud) & (_PAGE_PSE|_PAGE_DEVMAP)) == _PAGE_PSE;
+	return (pud_val(pud) & (_PAGE_PSE|_PAGE_DEVMAP|_PAGE_SPECIAL)) == _PAGE_PSE;
 }
 #endif
 
@@ -300,6 +300,17 @@ static inline int pmd_special(pmd_t pmd)
 {
 	return !!(pmd_flags(pmd) & _PAGE_SPECIAL);
 }
+
+#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
+static inline int pud_special(pud_t pud)
+{
+	return !!(pud_flags(pud) & _PAGE_SPECIAL);
+}
+#else
+static inline int pud_special(pud_t pud)
+{
+	return 0;
+}
 #endif
 
 #endif
@@ -487,6 +498,11 @@ static inline pud_t pud_mkhuge(pud_t pud)
 	return pud_set_flags(pud, _PAGE_PSE);
 }
 
+static inline pud_t pud_mkspecial(pud_t pud)
+{
+	return pud_set_flags(pud, _PAGE_SPECIAL);
+}
+
 static inline pud_t pud_mkyoung(pud_t pud)
 {
 	return pud_set_flags(pud, _PAGE_ACCESSED);
diff --git a/mm/gup.c b/mm/gup.c
index ba5f10535392..ae4abe5878ad 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2123,6 +2123,9 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
 		return __gup_device_huge_pud(orig, pudp, addr, end, pages, nr);
 	}
 
+	if (pud_special(orig))
+		return 0;
+
 	refs = 0;
 	page = pud_page(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
 	do {
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 06ad4d6f7477..cff707163bc1 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -879,6 +879,8 @@ static void insert_pfn_pud(struct vm_area_struct *vma, unsigned long addr,
 	entry = pud_mkhuge(pfn_t_pud(pfn, prot));
 	if (pfn_t_devmap(pfn))
 		entry = pud_mkdevmap(entry);
+	else if (pfn_t_special(pfn))
+		entry = pud_mkspecial(entry);
 	if (write) {
 		entry = pud_mkyoung(pud_mkdirty(entry));
 		entry = maybe_pud_mkwrite(entry, vma);
@@ -901,8 +903,7 @@ vm_fault_t vmf_insert_pfn_pud(struct vm_fault *vmf, pfn_t pfn, bool write)
 	 * but we need to be consistent with PTEs and architectures that
 	 * can't support a 'special' bit.
 	 */
-	BUG_ON(!(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP)) &&
-			!pfn_t_devmap(pfn));
+	BUG_ON(!(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP)));
 	BUG_ON((vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP)) ==
 						(VM_PFNMAP|VM_MIXEDMAP));
 	BUG_ON((vma->vm_flags & VM_PFNMAP) && is_cow_mapping(vma->vm_flags));
@@ -2031,7 +2032,8 @@ spinlock_t *__pud_trans_huge_lock(pud_t *pud, struct vm_area_struct *vma)
 	spinlock_t *ptl;
 
 	ptl = pud_lock(vma->vm_mm, pud);
-	if (likely(pud_trans_huge(*pud) || pud_devmap(*pud)))
+	if (likely(pud_trans_huge(*pud) || pud_devmap(*pud)) ||
+		   pud_special(*pud))
 		return ptl;
 	spin_unlock(ptl);
 	return NULL;
diff --git a/mm/memory.c b/mm/memory.c
index db99684d2cb3..109643219e1b 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1201,7 +1201,8 @@ static inline unsigned long zap_pud_range(struct mmu_gather *tlb,
 	pud = pud_offset(p4d, addr);
 	do {
 		next = pud_addr_end(addr, end);
-		if (pud_trans_huge(*pud) || pud_devmap(*pud)) {
+		if (pud_trans_huge(*pud) || pud_devmap(*pud) ||
+		    pud_special(*pud)) {
 			if (next - addr != HPAGE_PUD_SIZE) {
 				VM_BUG_ON_VMA(!rwsem_is_locked(&tlb->mm->mmap_sem), vma);
 				split_huge_pud(vma, pud, addr);
-- 
2.17.1