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

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Date:   Tue, 21 Nov 2017 19:59:54 +0000
From:   Roman Gushchin <guro@...com>
To:     Andrew Morton <akpm@...ux-foundation.org>
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, Nov 21, 2017 at 11:19:07AM -0800, Andrew Morton wrote:
> 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);
> 	
> ?

The idea was that the local variable guarantees the consistency
between Hugetlb and HugePages_Total numbers. Otherwise we have
to take hugetlb_lock.

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.

Thanks!