[patch 02/17] mm/gup_benchmark: fix MAP_HUGETLB case

[patch 02/17] mm/gup_benchmark: fix MAP_HUGETLB case

[akpm]

From: John Hubbard <jhubbard@nvidia.com>
Subject: mm/gup_benchmark: fix MAP_HUGETLB case

The MAP_HUGETLB ("-H" option) of gup_benchmark fails:

$ sudo ./gup_benchmark -H
mmap: Invalid argument

This is because gup_benchmark.c is passing in a file descriptor to mmap(),
but the fd came from opening up the /dev/zero file.  This confuses the
mmap syscall implementation, which thinks that, if the caller did not
specify MAP_ANONYMOUS, then the file must be a huge page file.  So it
attempts to verify that the file really is a huge page file, as you can
see here:

ksys_mmap_pgoff()
{
    if (!(flags & MAP_ANONYMOUS)) {
        retval = -EINVAL;
        if (unlikely(flags & MAP_HUGETLB && !is_file_hugepages(file)))
            goto out_fput; /* THIS IS WHERE WE END UP */

    else if (flags & MAP_HUGETLB) {
        ...proceed normally, /dev/zero is ok here...

...and of course is_file_hugepages() returns "false" for the /dev/zero
file.

The problem is that the user space program, gup_benchmark.c, really just
wants anonymous memory here.  The simplest way to get that is to pass
MAP_ANONYMOUS whenever MAP_HUGETLB is specified, so that's what this patch
does.

Link: http://lkml.kernel.org/r/20191021212435.398153-2-jhubbard@nvidia.com
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
Cc: Keith Busch <keith.busch@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 tools/testing/selftests/vm/gup_benchmark.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/tools/testing/selftests/vm/gup_benchmark.c~mm-gup_benchmark-fix-map_hugetlb-case
+++ a/tools/testing/selftests/vm/gup_benchmark.c
@@ -71,7 +71,7 @@ int main(int argc, char **argv)
 			flags |= MAP_SHARED;
 			break;
 		case 'H':
-			flags |= MAP_HUGETLB;
+			flags |= (MAP_HUGETLB | MAP_ANONYMOUS);
 			break;
 		default:
 			return -1;
_