[LTP] Question on hugemmap34

[Wei Gao]

Hi ALL

Is there any special config needed for this test case? Since the test failed with the following output on my test setup(opensuse15.5 with 6.12 kernel):

tst_hugepage.c:84: TINFO: 1 hugepage(s) reserved
tst_tmpdir.c:317: TINFO: Using /tmp/LTP_hugLSJb7r as tmpdir (btrfs filesystem)
tst_test.c:1100: TINFO: Mounting none to /tmp/LTP_hugLSJb7r/hugetlbfs fstyp=hugetlbfs flags=0
tst_test.c:1890: TINFO: LTP version: 20240930
tst_test.c:1894: TINFO: Tested kernel: 6.12.3-lp155.11.g78b0030-vanilla #1 SMP Fri Dec  6 08:56:39 UTC 2024 (78b0030) ppc64le
tst_test.c:1727: TINFO: Timeout per run is 0h 00m 30s
tst_coredump.c:32: TINFO: Avoid dumping corefile for process(pid=6671)
hugemmap34.c:88: TBROK: waitpid(0,0x7fffd8baa220,0) failed: ECHILD (10)
hugemmap34.c:92: TFAIL: Child: exited with 2

The root cause is the mmap call encounter failure with EBUSY.

LTP mmap call 
https://github.com/linux-test-project/ltp/blob/7bb960cc4f736d8860b6b266119e71e761e22b32/testcases/kernel/mem/hugetlb/hugemmap/hugemmap34.c#L71

hit kernel code
https://elixir.bootlin.com/linux/v6.12/source/arch/powerpc/mm/book3s64/slice.c#L568
  
Let me give an example to explain why this happens base pmap of process in my test system:

Address           Kbytes     RSS     PSS   Dirty    Swap Mode  Mapping
0000000010000000     256     256     128     256       0 r-xp- /root/ltp/testcases/kernel/mem/hugetlb/hugemmap/hugemmap34
0000000010040000      64      64      32      64       0 r--p- /root/ltp/testcases/kernel/mem/hugetlb/hugemmap/hugemmap34
0000000010050000      64      64      64      64       0 rw-p- /root/ltp/testcases/kernel/mem/hugetlb/hugemmap/hugemmap34
0000000010060000      64      64      64      64       0 rw-p-   [ anon ]
0000010010090000     192      64      64      64       0 rw-p-   [ anon ]
00007fff8f3b0000    2368    1408       0       0       0 r-xp- /lib64/libc.so.6
00007fff8f600000      64      64      32      64       0 r--p- /lib64/libc.so.6
00007fff8f610000      64      64      64      64       0 rw-p- /lib64/libc.so.6
00007fff8f620000      64      64      32      64       0 rw-s- /dev/shm/ltp_hugemmap34_15513 (deleted)   
00007fff8f630000     128       0       0       0       0 r--p-   [ anon ]
00007fff8f650000      64       0       0       0       0 r-xp-   [ anon ]
00007fff8f660000     320     128       0       0       0 r-xp- /lib64/ld64.so.2
00007fff8f6b0000      64      64      32      64       0 r--p- /lib64/ld64.so.2
00007fff8f6c0000      64      64      64      64       0 rw-p- /lib64/ld64.so.2
00007fffc6740000     192      64      64      64       0 rw-p-   [ stack ] <<<<
---------------- ------- ------- ------- ------- -------
total kB           20416    2432    1318     896       0

Test power system config:
#getconf PAGE_SIZE
65536
#grep Hugepagesize /proc/meminfo
Hugepagesize:      16384 kB

Kenel split VM space into: 
16 low_slice(64KB page size), each slice size is 256MB
4096 high_slice(64K page size), each slice size is 1TB

00007fffc6740000(stack) is belong 127th high_slice(range is 00007f0000000000 - 00007fffffffffff)

When mmap try to allocate a 16M space(with page size MMU_PAGE_16M) near stack address(00007fffc6740000), 
kernel will first check good_mask but failed, since the all slices is MMU_PAGE_64K 
but mmap request page size is MMU_PAGE_16M.
https://elixir.bootlin.com/linux/v6.12/source/arch/powerpc/mm/book3s64/slice.c#L531

Next kernel start check potential_mask(seach all slice which has no any VM mapped), obviously
the 127th slice is occupied and 126th is the good candidate one but address range not match 
the mmap's request address(0x00007fffc6740000 - 2 * hpage_size), so finally EBUSY returned. 
https://elixir.bootlin.com/linux/v6.12/source/arch/powerpc/mm/book3s64/slice.c#L559

BTW: I have tested a scenario which disables MAP_FIXED_NOREPLACE, the kernel will
allocate successfully within the range of 126th high slice. 

Thanks.
Regards
Wei Gao