Re: [PATCH 4/5] mm: extract code to fault in a page from __get_user_pages()

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 22 Apr 2014 03:50:36 +0300 "Kirill A. Shutemov" <kirill@shutemov.name> wrote:

> On Mon, Apr 21, 2014 at 04:35:22PM -0700, Andrew Morton wrote:
> > > +				ret = faultin_page(tsk, vma, start, &foll_flags,
> > > +						nonblocking);
> > > +				switch (ret) {
> > > +				case 0:
> > > +					break;
> > > +				case -EFAULT:
> > > +				case -ENOMEM:
> > > +				case -EHWPOISON:
> > > +					return i ? i : ret;
> > > +				case -EBUSY:
> > >  					return i;
> > > +				case -ENOENT:
> > > +					goto next_page;
> > > +				default:
> > > +					BUILD_BUG();
> > 
> > hm, why the BUILD_BUG?
> 
> To be sure that we can catch and handle any value faultin_page() can
> return.

Well sure, but to do that we use BUG().

BUILD_BUG() will fail to build and that of course is what happened.

> Could you show resulting faultin_page() from you tree? Or I can just
> rebase it on top of your tree once it will be published if you wish.

It looked like this:

				ret = faultin_page(tsk, vma, start, &foll_flags,
						nonblocking);
				switch (ret) {
				case 0:
					break;
				case -EFAULT:
				case -ENOMEM:
				case -EHWPOISON:
					return i ? i : ret;
				case -EBUSY:
					return i;
				case -ENOENT:
					goto next_page;
				default:
					BUILD_BUG();
				}

is that what you tested?


I suspect what happened is that your gcc worked out that faultin_page()
cannot return anything other than one of those six values and so the
compiler elided the BUILD_BUG() code.  But my gcc-4.4.4 isn't that smart.

For example this:

--- a/fs/open.c~a
+++ a/fs/open.c
@@ -1101,3 +1101,9 @@ int nonseekable_open(struct inode *inode
 }
 
 EXPORT_SYMBOL(nonseekable_open);
+
+void foo(void)
+{
+	if (0)
+		BUILD_BUG();
+}

compiles OK with gcc-4.4.4.

No matter, let's just leave it as a BUG().

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>