linux-kernel - Re: [PATCH] proc: mm: export PTE sizes directly in smaps (v2)

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Date:   Tue, 29 Nov 2016 09:01:17 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Dave Hansen <dave@...1.net>, linux-kernel@...r.kernel.org
Cc:     hch@....de, akpm@...ux-foundation.org, dan.j.williams@...el.com,
        khandual@...ux.vnet.ibm.com, linux-mm@...ck.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH] proc: mm: export PTE sizes directly in smaps (v2)

On 11/28/2016 10:39 PM, Dave Hansen wrote:
> ... cc'ing the arm64 maintainers
> 
> On 11/28/2016 01:07 PM, Vlastimil Babka wrote:
>> On 11/28/2016 05:52 PM, Dave Hansen wrote:
>>> On 11/24/2016 06:22 AM, Vlastimil Babka wrote:
>>>> On 11/17/2016 01:28 AM, Dave Hansen wrote:
>>>>> @@ -702,11 +707,13 @@ static int smaps_hugetlb_range(pte_t *pt
>>>>>      }
>>>>>      if (page) {
>>>>>          int mapcount = page_mapcount(page);
>>>>> +        unsigned long hpage_size = huge_page_size(hstate_vma(vma));
>>>>>
>>>>> +        mss->rss_pud += hpage_size;
>>>>
>>>> This hardcoded pud doesn't look right, doesn't the pmd/pud depend on
>>>> hpage_size?
>>>
>>> Urg, nope.  Thanks for noticing that!  I think we'll need something
>>> along the lines of:
>>>
>>>                 if (hpage_size == PUD_SIZE)
>>>                         mss->rss_pud += PUD_SIZE;
>>>                 else if (hpage_size == PMD_SIZE)
>>>                         mss->rss_pmd += PMD_SIZE;
>>
>> Sounds better, although I wonder whether there are some weird arches
>> supporting hugepage sizes that don't match page table levels. I recall
>> that e.g. MIPS could do arbitrary size, but dunno if the kernel supports
>> that...
> 
> arm64 seems to have pretty arbitrary sizes, and seems to be able to
> build them out of multiple hardware PTE sizes.  I think I can fix my
> code to handle those:
> 
>                 if (hpage_size >= PGD_SIZE)
>                         mss->rss_pgd += PGD_SIZE;
>                 else if (hpage_size >= PUD_SIZE)
>                         mss->rss_pud += PUD_SIZE;
>                 else if (hpage_size >= PMD_SIZE)
>                         mss->rss_pmd += PMD_SIZE;
>                 else
>                         mss->rss_pte += PAGE_SIZE;
> 
> But, I *think* that means that smaps_hugetlb_range() is *currently*
> broken for these intermediate arm64 sizes.  The code does:
> 
>                 if (mapcount >= 2)
>                         mss->shared_hugetlb += hpage_size;
>                 else
>                         mss->private_hugetlb += hpage_size;
> 
> So I *think* if we may count a hugetlbfs arm64 CONT_PTES page multiple
> times, and account hpage_size for *each* of the CONT_PTES.  That would
> artificially inflate the smaps output for those pages.

Hmm IIUC walk_hugetlb_range() will call the smaps_hugetlb_range()
callback once per hugepage, not once per "pte", no? See
hugetlb_entry_end(). In that case the current code should be OK and
yours would undercount?

> Will / Catalin, is there something I'm missing?
>