From: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
To: bpf@vger.kernel.org
Cc: Andrii Nakryiko <andriin@fb.com>, Jiri Olsa <jolsa@redhat.com>,
Jiri Benc <jbenc@redhat.com>
Subject: selftests: bpf: mmap question
Date: Thu, 23 Jul 2020 14:02:02 +0300 [thread overview]
Message-ID: <xunyft9i1olx.fsf@redhat.com> (raw)
Hi!
I have a question about the part of the test:
/* check some more advanced mmap() manipulations */
/* map all but last page: pages 1-3 mapped */
tmp1 = mmap(NULL, 3 * page_size, PROT_READ, MAP_SHARED,
data_map_fd, 0);
if (CHECK(tmp1 == MAP_FAILED, "adv_mmap1", "errno %d\n", errno))
goto cleanup;
/* unmap second page: pages 1, 3 mapped */
err = munmap(tmp1 + page_size, page_size);
if (CHECK(err, "adv_mmap2", "errno %d\n", errno)) {
munmap(tmp1, map_sz);
goto cleanup;
}
/* map page 2 back */
tmp2 = mmap(tmp1 + page_size, page_size, PROT_READ,
MAP_SHARED | MAP_FIXED, data_map_fd, 0);
if (CHECK(tmp2 == MAP_FAILED, "adv_mmap3", "errno %d\n", errno)) {
munmap(tmp1, page_size);
munmap(tmp1 + 2*page_size, page_size);
goto cleanup;
}
CHECK(tmp1 + page_size != tmp2, "adv_mmap4",
"tmp1: %p, tmp2: %p\n", tmp1, tmp2);
/* re-map all 4 pages */
tmp2 = mmap(tmp1, 4 * page_size, PROT_READ, MAP_SHARED | MAP_FIXED,
data_map_fd, 0);
if (CHECK(tmp2 == MAP_FAILED, "adv_mmap5", "errno %d\n", errno)) {
munmap(tmp1, 3 * page_size); /* unmap page 1 */
goto cleanup;
}
CHECK(tmp1 != tmp2, "adv_mmap6", "tmp1: %p, tmp2: %p\n", tmp1, tmp2);
In my configuration the first mapping
/* map all but last page: pages 1-3 mapped */
tmp1 = mmap(NULL, 3 * page_size, PROT_READ, MAP_SHARED,
data_map_fd, 0);
maps the area to the 3 pages right before the TLS page.
I find it's pretty ok.
But then the 4 page mapping
/* re-map all 4 pages */
tmp2 = mmap(tmp1, 4 * page_size, PROT_READ, MAP_SHARED | MAP_FIXED,
data_map_fd, 0);
since it has MAP_FIXED flag, unmaps TLS and maps the former TLS
address BPF map.
Which is again exactly the behaviour of MAP_FIXED, but it breaks
the test.
Using MAP_FIXED_NOREPLACE fails the check:
CHECK(tmp1 != tmp2, "adv_mmap6", "tmp1: %p, tmp2: %p\n", tmp1, tmp2);
as expected.
Should the test be modified to be a bit more relaxed? Since the
kernel behaviour looks correct or I'm missing something?
PS: BTW, the previous data_map mapping left unmmaped. Is it expected?
--
WBR,
Yauheni Kaliuta
next reply other threads:[~2020-07-23 11:02 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-23 11:02 Yauheni Kaliuta [this message]
2020-07-28 5:15 ` selftests: bpf: mmap question Andrii Nakryiko
2020-08-10 15:31 ` Yauheni Kaliuta
2020-08-11 0:27 ` Andrii Nakryiko
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