[PATCH v3 3/9] mm: Split huge pages on write-notify or COW

[Thomas Hellström (VMware)]

From: Thomas Hellstrom <thellstrom at vmware.com>

We currently only do COW and write-notify on the PTE level, so if the
huge_fault() handler returns VM_FAULT_FALLBACK on wp faults,
split the huge pages and page-table entries. Also do this for huge PUDs
if there is no huge_fault() handler and the vma is not anonymous, similar
to how it's done for PMDs.

Note that fs/dax.c does the splitting in the huge_fault() handler, but as
huge_fault() is implemented by modules we need to consider whether to
export the splitting functions for use in the modules or whether to try
to keep calls in the core. Opt for the latter. A follow-up patch can
remove the dax.c split_huge_pmd() if needed.

Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Michal Hocko <mhocko at suse.com>
Cc: "Matthew Wilcox (Oracle)" <willy at infradead.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov at linux.intel.com>
Cc: Ralph Campbell <rcampbell at nvidia.com>
Cc: "Jérôme Glisse" <jglisse at redhat.com>
Cc: "Christian König" <christian.koenig at amd.com>
Cc: Dan Williams <dan.j.williams at intel.com>
Signed-off-by: Thomas Hellstrom <thellstrom at vmware.com>
---
 mm/memory.c | 27 +++++++++++++++++++--------
 1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 17aadc751e5c..4c49fe963e5c 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3880,11 +3880,14 @@ static inline vm_fault_t wp_huge_pmd(struct vm_fault *vmf, pmd_t orig_pmd)
 {
 	if (vma_is_anonymous(vmf->vma))
 		return do_huge_pmd_wp_page(vmf, orig_pmd);
-	if (vmf->vma->vm_ops->huge_fault)
-		return vmf->vma->vm_ops->huge_fault(vmf, PE_SIZE_PMD);
+	if (vmf->vma->vm_ops->huge_fault) {
+		vm_fault_t ret = vmf->vma->vm_ops->huge_fault(vmf, PE_SIZE_PMD);
 
-	/* COW handled on pte level: split pmd */
-	VM_BUG_ON_VMA(vmf->vma->vm_flags & VM_SHARED, vmf->vma);
+		if (!(ret & VM_FAULT_FALLBACK))
+			return ret;
+	}
+
+	/* COW or write-notify handled on pte level: split pmd. */
 	__split_huge_pmd(vmf->vma, vmf->pmd, vmf->address, false, NULL);
 
 	return VM_FAULT_FALLBACK;
@@ -3897,12 +3900,20 @@ static inline bool vma_is_accessible(struct vm_area_struct *vma)
 
 static vm_fault_t create_huge_pud(struct vm_fault *vmf)
 {
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+#if defined(CONFIG_TRANSPARENT_HUGEPAGE) &&			\
+	defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)
 	/* No support for anonymous transparent PUD pages yet */
 	if (vma_is_anonymous(vmf->vma))
-		return VM_FAULT_FALLBACK;
-	if (vmf->vma->vm_ops->huge_fault)
-		return vmf->vma->vm_ops->huge_fault(vmf, PE_SIZE_PUD);
+		goto split;
+	if (vmf->vma->vm_ops->huge_fault) {
+		vm_fault_t ret = vmf->vma->vm_ops->huge_fault(vmf, PE_SIZE_PUD);
+
+		if (!(ret & VM_FAULT_FALLBACK))
+			return ret;
+	}
+split:
+	/* COW or write-notify not handled on PUD level: split pud.*/
+	__split_huge_pud(vmf->vma, vmf->pud, vmf->address);
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 	return VM_FAULT_FALLBACK;
 }
-- 
2.21.1