linux-kernel - Re: [PATCH v2] mm: show total hugetlb memory consumption in /proc/meminfo

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Date:   Tue, 21 Nov 2017 11:19:07 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Roman Gushchin <guro@...com>
Cc:     <linux-mm@...ck.org>, Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        David Rientjes <rientjes@...gle.com>, <kernel-team@...com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] mm: show total hugetlb memory consumption in
 /proc/meminfo

On Tue, 21 Nov 2017 15:15:55 +0000 Roman Gushchin <guro@...com> wrote:

> > > +
> > > +	for_each_hstate(h) {
> > > +		unsigned long count = h->nr_huge_pages;
> > > +
> > > +		total += (PAGE_SIZE << huge_page_order(h)) * count;
> > > +
> > > +		if (h == &default_hstate)
> > 
> > I'm not understanding this test.  Are we assuming that default_hstate
> > always refers to the highest-index hstate?  If so why, and is that
> > valid?
> 
> As Mike and Michal pointed, default_hstate is defined as
>   #define default_hstate (hstates[default_hstate_idx]),
> where default_hstate_idx can be altered by a boot argument.
> 
> We're iterating over all states to calculate total and also
> print some additional info for the default size. Having a single
> loop guarantees consistency of these numbers.
> 

OK, I misread the handling of `count' -> HugePages_Total.

It seems unnecessarily obscure?

	for_each_hstate(h) {
		unsigned long count = h->nr_huge_pages;

		total += (PAGE_SIZE << huge_page_order(h)) * count;

		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",
				   count,
				   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);


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);
	
?