mm/vmalloc: use physical page count in vread_iter() for VM_ALLOC areas

For VM_ALLOC areas in vread_iter(), derive the vm area size from
vm->nr_pages rather than get_vm_area_size().

Only VM_ALLOC areas are subject to vrealloc() shrinking, which frees pages
without reducing the virtual reservation size.  Switch to using
vm->nr_pages for VM_ALLOC areas so the reader remains correct once shrink
support is added.  Other mapping types (vmap, ioremap) do not initialize
nr_pages and will continue using get_vm_area_size().

[[email protected]: add an nr_pages check]
  Link: https://lore.kernel.org/[email protected]
Link: https://lore.kernel.org/[email protected]
Signed-off-by: Shivam Kalra <[email protected]>
Reviewed-by: Uladzislau Rezki (Sony) <[email protected]>
Cc: Alice Ryhl <[email protected]>
Cc: Danilo Krummrich <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/vmalloc.c

@@ -4666,7 +4666,18 @@ long vread_iter(struct iov_iter *iter, const char *addr, size_t count)
 		smp_rmb();
 
 		vaddr = (char *) va->va_start;
-		size = vm ? get_vm_area_size(vm) : va_size(va);
+		if (vm)
+			/*
+			 * For VM_ALLOC areas, use nr_pages rather than
+			 * get_vm_area_size() because vrealloc() may shrink
+			 * the mapping without updating area->size. Other
+			 * mapping types (vmap, ioremap) don't set nr_pages.
+			 */
+			size = (vm->flags & VM_ALLOC && vm->nr_pages) ?
+				       (vm->nr_pages << PAGE_SHIFT) :
+				       get_vm_area_size(vm);
+		else
+			size = va_size(va);
 
 		if (addr >= vaddr + size)
 			goto next_va;