mm/gup_benchmark: handle gup failures

Patch series "mm/get_user_pages_fast fixes, cleanups", v2.

Turns out get_user_pages_fast and __get_user_pages_fast return different
values on error when given a single page: __get_user_pages_fast returns
0.  get_user_pages_fast returns either 0 or an error.

Callers of get_user_pages_fast expect an error so fix it up to return an
error consistently.

Stress the difference between get_user_pages_fast and
__get_user_pages_fast to make sure callers aren't confused.

This patch (of 3):

__gup_benchmark_ioctl does not handle the case where get_user_pages_fast
fails:

 - a negative return code will cause a buffer overrun

 - returning with partial success will cause use of uninitialized
   memory.

[akpm@linux-foundation.org: simplification]
Link: http://lkml.kernel.org/r/1522962072-182137-3-git-send-email-mst@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Thorsten Leemhuis <regressions@leemhuis.info>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/gup_benchmark.c

@@ -23,7 +23,7 @@ static int __gup_benchmark_ioctl(unsigned int cmd,
 	struct page **pages;
 
 	nr_pages = gup->size / PAGE_SIZE;
-	pages = kvmalloc(sizeof(void *) * nr_pages, GFP_KERNEL);
+	pages = kvzalloc(sizeof(void *) * nr_pages, GFP_KERNEL);
 	if (!pages)
 		return -ENOMEM;
 
@@ -41,6 +41,8 @@ static int __gup_benchmark_ioctl(unsigned int cmd,
 		}
 
 		nr = get_user_pages_fast(addr, nr, gup->flags & 1, pages + i);
+		if (nr <= 0)
+			break;
 		i += nr;
 	}
 	end_time = ktime_get();