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From: John Hubbard <jhubbard@nvidia.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Keith Busch <keith.busch@intel.com>,
	LKML <linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>,
	<linux-kselftest@vger.kernel.org>,
	John Hubbard <jhubbard@nvidia.com>
Subject: [PATCH 1/1] mm/gup_benchmark: fix MAP_HUGETLB case
Date: Mon, 21 Oct 2019 14:24:35 -0700	[thread overview]
Message-ID: <20191021212435.398153-2-jhubbard@nvidia.com> (raw)
In-Reply-To: <20191021212435.398153-1-jhubbard@nvidia.com>

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.

Cc: Keith Busch <keith.busch@intel.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 tools/testing/selftests/vm/gup_benchmark.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/vm/gup_benchmark.c b/tools/testing/selftests/vm/gup_benchmark.c
index cb3fc09645c4..485cf06ef013 100644
--- a/tools/testing/selftests/vm/gup_benchmark.c
+++ b/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;
-- 
2.23.0


  reply	other threads:[~2019-10-21 21:24 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-21 21:24 [PATCH 0/1] " John Hubbard
2019-10-21 21:24 ` John Hubbard [this message]
2019-10-22 17:14   ` [PATCH 1/1] " Jerome Glisse
2019-10-22 18:41     ` John Hubbard
2019-10-22 18:57       ` Jerome Glisse
2019-10-22 19:39         ` John Hubbard
2019-10-22 21:34           ` Andrew Morton

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