Re: THP: AnonHugePages in /proc/[pid]/smaps is correct or not?

[Zhouping Liu <zliu@redhat.com>]

On 04/02/2013 06:23 AM, David Rientjes wrote:
> On Mon, 1 Apr 2013, Zhouping Liu wrote:
>
>> 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.
>>
> It's not a problem with identifying an anonymous mapping as a hugepage,
> setting thp enabled to "always" does not guarantee that they will always
> be allocatable or that your mmap() will be 2MB aligned.  Your sample code
> is using mmap() instead of posix_memalign() so you'll probably only get
> 100% hugepages only 1/512th of the time.

I don't understand clearly the last sentence 'you'll probably only get 
100% hugepages only 1/512th of the time.'
could you please explain more details about 'only 1/512th of the time'?

>
>> 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
> "hg" would be shown in VmFlags if your MADV_HUGEPAGE was successful, are
> you sure this is the right vma?

I think it's the same issue as the above, according to the sample code, 
it does mmap() five times in total,
and each time it map 2MB anonymous maps, as all the maps maybe aren't 
2MB aligned, so "AnonHugePages"
show 0 kB, and no "hg" VmFalgs.

so, again, if I understand correctly, thp should tune the naturally 
aligned maps, such as generated by mmap()/malloc(),
make such maps 'hugepagesize' aligned if the maps or vma is equal and 
greater than 'hugepagesize', doesn't it?

Thanks,
Zhouping