mm/vmalloc.c: allow vread() to read out vm_map_ram areas

Currently, vread() can read out vmalloc areas which is associated with
a vm_struct. While this doesn't work for areas created by vm_map_ram()
interface because it doesn't allocate a vm_struct. Then in vread(),
these areas will be skipped.

Here, add a new function vb_vread() to read out areas managed by
vmap_block specifically. Then recognize vm_map_ram areas via vmap->flags
and handle  them respectively.

Stephen Brennan <stephen.s.brennan@oracle.com>
Signed-off-by: Baoquan He <bhe@redhat.com>
Link: https://lore.kernel.org/all/87ilk6gos2.fsf@oracle.com/T/#u

mm/vmalloc.c

@@ -3532,6 +3532,46 @@ static int aligned_vread(char *buf, char *addr, unsigned long count)
 	return copied;
 }
 
+static void vb_vread(char *buf, char *addr, int count)
+{
+	char *start;
+	struct vmap_block *vb;
+	unsigned long offset;
+	unsigned int rs, re, n;
+
+	offset = ((unsigned long)addr & (VMAP_BLOCK_SIZE - 1)) >> PAGE_SHIFT;
+	vb = xa_load(&vmap_blocks, addr_to_vb_idx((unsigned long)addr));
+
+	spin_lock(&vb->lock);
+	if (bitmap_empty(vb->used_map, VMAP_BBMAP_BITS)) {
+		spin_unlock(&vb->lock);
+		memset(buf, 0, count);
+		return;
+	}
+	for_each_set_bitrange(rs, re, vb->used_map, VMAP_BBMAP_BITS) {
+		if (!count)
+			break;
+		start = vmap_block_vaddr(vb->va->va_start, rs);
+		if (addr < start) {
+			if (count == 0)
+				break;
+			*buf = '\0';
+			buf++;
+			addr++;
+			count--;
+		}
+		n = (re - rs + 1) << PAGE_SHIFT;
+		if (n > count)
+			n = count;
+		aligned_vread(buf, start, n);
+
+		buf += n;
+		addr += n;
+		count -= n;
+	}
+	spin_unlock(&vb->lock);
+}
+
 /**
  * vread() - read vmalloc area in a safe way.
  * @buf:     buffer for reading data
@@ -3562,7 +3602,7 @@ long vread(char *buf, char *addr, unsigned long count)
 	struct vm_struct *vm;
 	char *vaddr, *buf_start = buf;
 	unsigned long buflen = count;
-	unsigned long n;
+	unsigned long n, size;
 
 	addr = kasan_reset_tag(addr);
 
@@ -3583,12 +3623,14 @@ long vread(char *buf, char *addr, unsigned long count)
 		if (!count)
 			break;
 
-		if (!va->vm)
+		if (!(va->flags & VMAP_RAM) && !va->vm)
 			continue;
 
 		vm = va->vm;
-		vaddr = (char *) vm->addr;
-		if (addr >= vaddr + get_vm_area_size(vm))
+		vaddr = (char *) va->va_start;
+		size = vm ? get_vm_area_size(vm) : va_size(va);
+
+		if (addr >= vaddr + size)
 			continue;
 		while (addr < vaddr) {
 			if (count == 0)
@@ -3598,10 +3640,13 @@ long vread(char *buf, char *addr, unsigned long count)
 			addr++;
 			count--;
 		}
-		n = vaddr + get_vm_area_size(vm) - addr;
+		n = vaddr + size - addr;
 		if (n > count)
 			n = count;
-		if (!(vm->flags & VM_IOREMAP))
+
+		if ((va->flags & (VMAP_RAM|VMAP_BLOCK)) == (VMAP_RAM|VMAP_BLOCK))
+			vb_vread(buf, addr, n);
+		else if ((va->flags & VMAP_RAM) || !(vm->flags & VM_IOREMAP))
 			aligned_vread(buf, addr, n);
 		else /* IOREMAP area is treated as memory hole */
 			memset(buf, 0, n);