[PATCH 2/4] mm: introduce new VM_NOZEROPAGE flag

Dominik Dingel
2014-10-17T14:09:48Z

Add a new vma flag to allow an architecture to disable the backing
of non-present, anonymous pages with the read-only empty zero page.

Signed-off-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
---
include/linux/mm.h | 13 +++++++++++--
mm/huge_memory.c | 2 +-
mm/memory.c | 2 +-
3 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index cd33ae2..8f09c91 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -113,7 +113,7 @@ extern unsigned int kobjsize(const void *objp);
#define VM_GROWSDOWN 0x00000100 /* general info on the segment */
#define VM_PFNMAP 0x00000400 /* Page-ranges managed without "struct page", just pure PFN */
#define VM_DENYWRITE 0x00000800 /* ETXTBSY on write attempts.. */
-
+#define VM_NOZEROPAGE 0x00001000 /* forbid new zero page mappings */
#define VM_LOCKED 0x00002000
#define VM_IO 0x00004000 /* Memory mapped I/O or similar */

@@ -179,7 +179,7 @@ extern unsigned int kobjsize(const void *objp);
#define VM_SPECIAL (VM_IO | VM_DONTEXPAND | VM_PFNMAP | VM_MIXEDMAP)

/* This mask defines which mm->def_flags a process can inherit its parent */
-#define VM_INIT_DEF_MASK VM_NOHUGEPAGE
+#define VM_INIT_DEF_MASK (VM_NOHUGEPAGE | VM_NOZEROPAGE)

/*
* mapping from the currently active vm_flags protection bits (the
@@ -1293,6 +1293,15 @@ static inline int stack_guard_page_end(struct vm_area_struct *vma,
!vma_growsup(vma->vm_next, addr);
}

+static inline int vma_forbids_zeropage(struct vm_area_struct *vma)
+{
+#ifdef CONFIG_NOZEROPAGE
+ return vma->vm_flags & VM_NOZEROPAGE;
+#else
+ return 0;
+#endif
+}
+
extern struct task_struct *task_of_stack(struct task_struct *task,
struct vm_area_struct *vma, bool in_group);

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index de98415..c271265 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -805,7 +805,7 @@ int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
return VM_FAULT_OOM;
if (unlikely(khugepaged_enter(vma, vma->vm_flags)))
return VM_FAULT_OOM;
- if (!(flags & FAULT_FLAG_WRITE) &&
+ if (!(flags & FAULT_FLAG_WRITE) && !vma_forbids_zeropage(vma) &&
transparent_hugepage_use_zero_page()) {
spinlock_t *ptl;
pgtable_t pgtable;
diff --git a/mm/memory.c b/mm/memory.c
index 64f82aa..1859b2b 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2640,7 +2640,7 @@ static int do_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
return VM_FAULT_SIGBUS;

/* Use the zero-page for reads */
- if (!(flags & FAULT_FLAG_WRITE)) {
+ if (!(flags & FAULT_FLAG_WRITE) && !vma_forbids_zeropage(vma)) {
entry = pte_mkspecial(pfn_pte(my_zero_pfn(address),
vma->vm_page_prot));
page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
--
1.8.5.5