linux-kernel - THP: AnonHugePages in /proc/[pid]/smaps is correct or not?

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Date:	Mon, 1 Apr 2013 04:00:27 -0400 (EDT)
From:	Zhouping Liu <zliu@...hat.com>
To:	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>, Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Amos Kong <akong@...hat.com>
Subject: THP: AnonHugePages in /proc/[pid]/smaps is correct or not?

Hi all,

I found THP can't correctly distinguish one anonymous hugepage map.

1. when /sys/kernel/mm/transparent_hugepage/enabled is 'always', the
   amount of THP always is one less.

   Testing code:
---- snip --------
unsigned long hugepagesize = (1UL << 21);

int main()
{
	void *addr;
	int i;

	printf("pid is %d\n", getpid());

	for (i = 0; i < 5; i++) {
		addr = mmap(NULL, hugepagesize, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANON, -1, 0);

		if (addr == MAP_FAILED) {
			perror("mmap");
			return -1;
		}

		memset(addr, i, hugepagesize);
	}

	sleep(50);

	return 0;
}
------ snip ----------

the /proc/[pid]/smaps show that Anonymous is 10240kB but AnonHugePages is 8192Kb, one THP less:
----- snip --------
7f59ccc01000-7f59cd601000 rw-p 00000000 00:00 0
Size:              10240 kB
Rss:               10240 kB
Pss:               10240 kB
Shared_Clean:          0 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:     10240 kB
Referenced:        10240 kB
Anonymous:         10240 kB
AnonHugePages:      8192 kB
Swap:                  0 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Locked:                0 kB
VmFlags: rd wr mr mw me ac hg
------- sinp ---------

2. when /sys/kernel/mm/transparent_hugepage/enabled is 'madvise', THP can't
   distinguish any one anonymous hugepage size:

   Testing code:
-------- snip --------
unsigned long hugepagesize = (1UL << 21);

int main()
{
	void *addr;
	int i;

	printf("pid is %d\n", getpid());

	for (i = 0; i < 5; i++) {
		addr = mmap(NULL, hugepagesize, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANON, -1, 0);

		if (addr == MAP_FAILED) {
			perror("mmap");
			return -1;
		}

		if (madvise(addr, hugepagesize, MADV_HUGEPAGE) == -1) {
			perror("madvise");
			return -1;
		}

		memset(addr, i, hugepagesize);
	}

	sleep(50);

	return 0;
}
--------- snip ----------

The result is that it can't find any AnonHugePages from /proc/[pid]/smaps :
-------------- snip -------
7f0b38cd0000-7f0b396d0000 rw-p 00000000 00:00 0
Size:              10240 kB
Rss:               10240 kB
Pss:               10240 kB
Shared_Clean:          0 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:     10240 kB
Referenced:        10240 kB
Anonymous:         10240 kB
AnonHugePages:         0 kB
Swap:                  0 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Locked:                0 kB
VmFlags: rd wr mr mw me ac
----------- snip ----------

3. when I made the address aligned with HUGEPAGESIZE using 'posix_memalign()' instead of mmap(),
   THP perform good, and can distinguish all anonymous huge pages.

my question is:
1. all the above behaviour is right?
2. why THP can't distinguish one naturally aligned huge page(generated by mmap())?

-- 
Thanks,
Zhouping
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