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

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Message-ID: <6262d9fa-8098-4e18-4129-932e5e4857cb@sr71.net>
Date:   Mon, 28 Nov 2016 13:39:49 -0800
From:   Dave Hansen <dave@...1.net>
To:     Vlastimil Babka <vbabka@...e.cz>, 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)

... 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.

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