[RFC PATCH 4/6] mm: Add COW PTE fallback function

[Chih-En Lin <shiyn.lin@xxxxxxxxx>]

The lifetime of COW PTE will handle by ownership and a reference count.
When the process wants to write the COW PTE, which reference count is 1,
it will reuse the COW PTE instead of copying then free.

Only the owner will update its RSS state and the record of page table
bytes allocation. So we need to handle when the non-owner process gets
the fallback COW PTE.

This commit prepares for the following implementation of the reference
count for COW PTE.

Signed-off-by: Chih-En Lin <shiyn.lin@xxxxxxxxx>
---
 mm/memory.c | 66 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 66 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index 76e3af9639d9..dcb678cbb051 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1000,6 +1000,34 @@ page_copy_prealloc(struct mm_struct *src_mm, struct vm_area_struct *vma,
 	return new_page;
 }
 
+static inline void cow_pte_rss(struct mm_struct *mm, struct vm_area_struct *vma,
+	pmd_t *pmdp, unsigned long addr, unsigned long end, bool inc_dec)
+{
+	int rss[NR_MM_COUNTERS];
+	pte_t *orig_ptep, *ptep;
+	struct page *page;
+
+	init_rss_vec(rss);
+
+	ptep = pte_offset_map(pmdp, addr);
+	orig_ptep = ptep;
+	arch_enter_lazy_mmu_mode();
+	do {
+		if (pte_none(*ptep) || pte_special(*ptep))
+			continue;
+
+		page = vm_normal_page(vma, addr, *ptep);
+		if (page) {
+			if (inc_dec)
+				rss[mm_counter(page)]++;
+			else
+				rss[mm_counter(page)]--;
+		}
+	} while (ptep++, addr += PAGE_SIZE, addr != end);
+	arch_leave_lazy_mmu_mode();
+	add_mm_rss_vec(mm, rss);
+}
+
 static int
 copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
 	       pmd_t *dst_pmd, pmd_t *src_pmd, unsigned long addr,
@@ -4554,6 +4582,44 @@ static vm_fault_t wp_huge_pud(struct vm_fault *vmf, pud_t orig_pud)
 	return VM_FAULT_FALLBACK;
 }
 
+/* COW PTE fallback to normal PTE:
+ * - two state here
+ *   - After break child :   [parent, rss=1, ref=1, write=NO , owner=parent]
+ *                        to [parent, rss=1, ref=1, write=YES, owner=NULL  ]
+ *   - After break parent:   [child , rss=0, ref=1, write=NO , owner=NULL  ]
+ *                        to [child , rss=1, ref=1, write=YES, owner=NULL  ]
+ */
+void cow_pte_fallback(struct vm_area_struct *vma, pmd_t *pmd,
+		unsigned long addr)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	unsigned long start, end;
+	pmd_t new;
+
+	BUG_ON(pmd_write(*pmd));
+
+	start = addr & PMD_MASK;
+	end = (addr + PMD_SIZE) & PMD_MASK;
+
+	/* If pmd is not owner, it needs to increase the rss.
+	 * Since only the owner has the RSS state for the COW PTE.
+	 */
+	if (!cow_pte_owner_is_same(pmd, pmd)) {
+		cow_pte_rss(mm, vma, pmd, start, end, true /* inc */);
+		mm_inc_nr_ptes(mm);
+		smp_wmb();
+		pmd_populate(mm, pmd, pmd_page(*pmd));
+	}
+
+	/* Reuse the pte page */
+	set_cow_pte_owner(pmd, NULL);
+	new = pmd_mkwrite(*pmd);
+	set_pmd_at(mm, addr, pmd, new);
+
+	BUG_ON(!pmd_write(*pmd));
+	BUG_ON(pmd_page(*pmd)->cow_pte_owner);
+}
+
 /*
  * These routines also need to handle stuff like marking pages dirty
  * and/or accessed for architectures that don't do it in hardware (most
-- 
2.36.1