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SubjectRe: [PATCH v2] mm: show total hugetlb memory consumption in /proc/meminfo
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On 11/21/2017 11:59 AM, Roman Gushchin wrote:
> On Tue, Nov 21, 2017 at 11:19:07AM -0800, Andrew Morton wrote:
>>
>> Why not
>>
>> seq_printf(m,
>> "HugePages_Total: %5lu\n"
>> "HugePages_Free: %5lu\n"
>> "HugePages_Rsvd: %5lu\n"
>> "HugePages_Surp: %5lu\n"
>> "Hugepagesize: %8lu kB\n",
>> h->nr_huge_pages,
>> h->free_huge_pages,
>> h->resv_huge_pages,
>> h->surplus_huge_pages,
>> 1UL << (huge_page_order(h) + PAGE_SHIFT - 10));
>>
>> for_each_hstate(h)
>> total += (PAGE_SIZE << huge_page_order(h)) * h->nr_huge_pages;
>> seq_printf(m, "Hugetlb: %8lu kB\n", total / 1024);
>>
>> ?
>
> The idea was that the local variable guarantees the consistency
> between Hugetlb and HugePages_Total numbers. Otherwise we have
> to take hugetlb_lock.

Most important it prevents HugePages_Total from being larger than
Hugetlb.

> What we can do, is to rename "count" into "nr_huge_pages", like:
>
> for_each_hstate(h) {
> unsigned long nr_huge_pages = h->nr_huge_pages;
>
> total += (PAGE_SIZE << huge_page_order(h)) * nr_huge_pages;
>
> if (h == &default_hstate)
> seq_printf(m,
> "HugePages_Total: %5lu\n"
> "HugePages_Free: %5lu\n"
> "HugePages_Rsvd: %5lu\n"
> "HugePages_Surp: %5lu\n"
> "Hugepagesize: %8lu kB\n",
> nr_huge_pages,
> h->free_huge_pages,
> h->resv_huge_pages,
> h->surplus_huge_pages,
> (PAGE_SIZE << huge_page_order(h)) / 1024);
> }
>
> seq_printf(m, "Hugetlb: %8lu kB\n", total / 1024);
>
> But maybe taking a lock is not a bad idea, because it will also
> guarantee consistency between other numbers (like HugePages_Free) as well,
> which is not true right now.

You are correct in that there is no consistency guarantee for the numbers
with the default huge page size today. However, I am not really a fan of
taking the lock for that guarantee. IMO, the above code is fine.

This discussion reminds me that ideally there should be a per-hstate lock.
My guess is that the global lock is a carry over from the days when only
a single huge page size was supported. In practice, I don't think this is
much of an issue as people typically only use a single huge page size. But,
if anyone thinks is/may be an issue I am happy to make the changes.

--
Mike Kravetz

>
> Thanks!
>
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