proc/smaps: show amount of nonlinear ptes in vma

Currently, nonlinear mappings can not be distinguished from ordinary
mappings. This patch adds into /proc/pid/smaps line "Nonlinear: <size>
kB", where size is amount of nonlinear ptes in vma, this line appears only
if VM_NONLINEAR is set. This information may be useful not only for
checkpoint/restore project.

Requested by Pavel Emelyanov.

Signed-off-by: Konstantin Khlebnikov <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Pavel Emelyanov <[e-mail]>
Cc: Alexey Dobriyan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/proc/task_mmu.c

···
 	unsigned long anonymous;
 	unsigned long anonymous_thp;
 	unsigned long swap;
+	unsigned long nonlinear;
 	u64 pss;
 };
 
···
 {
 	struct mem_size_stats *mss = walk->private;
 	struct vm_area_struct *vma = mss->vma;
+	pgoff_t pgoff = linear_page_index(vma, addr);
 	struct page *page = NULL;
 	int mapcount;
 
···
 			mss->swap += ptent_size;
 		else if (is_migration_entry(swpent))
 			page = migration_entry_to_page(swpent);
+	} else if (pte_file(ptent)) {
+		if (pte_to_pgoff(ptent) != pgoff)
+			mss->nonlinear += ptent_size;
 	}
 
 	if (!page)
···
 
 	if (PageAnon(page))
 		mss->anonymous += ptent_size;
+
+	if (page->index != pgoff)
+		mss->nonlinear += ptent_size;
 
 	mss->resident += ptent_size;
 	/* Accumulate the size in pages that have been accessed. */
···
 		   vma_mmu_pagesize(vma) >> 10,
 		   (vma->vm_flags & VM_LOCKED) ?
 			(unsigned long)(mss.pss >> (10 + PSS_SHIFT)) : 0);
+
+	if (vma->vm_flags & VM_NONLINEAR)
+		seq_printf(m, "Nonlinear:      %8lu kB\n",
+				mss.nonlinear >> 10);
 
 	if (m->count < m->size)  /* vma is copied successfully */
 		m->version = (vma != get_gate_vma(task->mm))