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

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Date:	Tue, 02 Apr 2013 20:26:47 +0800
From:	Simon Jeons <simon.jeons@...il.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	Zhouping Liu <zliu@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>, Mel Gorman <mgorman@...e.de>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Amos Kong <akong@...hat.com>
Subject: Re: THP: AnonHugePages in /proc/[pid]/smaps is correct or not?

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

Btw, why need 2MB aligned? Does it has relationship with tlb?

>   
> is using mmap() instead of posix_memalign() so you'll probably only get
> 100% hugepages 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?
>
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