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

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Date:   Tue, 21 Nov 2017 15:15:55 +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 Mon, Nov 20, 2017 at 04:51:10PM -0800, Andrew Morton wrote:
> On Wed, 15 Nov 2017 23:14:09 +0000 Roman Gushchin <guro@...com> wrote:
> 
> > Currently we display some hugepage statistics (total, free, etc)
> > in /proc/meminfo, but only for default hugepage size (e.g. 2Mb).
> > 
> > If hugepages of different sizes are used (like 2Mb and 1Gb on x86-64),
> > /proc/meminfo output can be confusing, as non-default sized hugepages
> > are not reflected at all, and there are no signs that they are
> > existing and consuming system memory.
> > 
> > To solve this problem, let's display the total amount of memory,
> > consumed by hugetlb pages of all sized (both free and used).
> > Let's call it "Hugetlb", and display size in kB to match generic
> > /proc/meminfo style.
> > 
> > For example, (1024 2Mb pages and 2 1Gb pages are pre-allocated):
> >   $ cat /proc/meminfo
> >   MemTotal:        8168984 kB
> >   MemFree:         3789276 kB
> >   <...>
> >   CmaFree:               0 kB
> >   HugePages_Total:    1024
> >   HugePages_Free:     1024
> >   HugePages_Rsvd:        0
> >   HugePages_Surp:        0
> >   Hugepagesize:       2048 kB
> >   Hugetlb:         4194304 kB
> >   DirectMap4k:       32632 kB
> >   DirectMap2M:     4161536 kB
> >   DirectMap1G:     6291456 kB
> > 
> > Also, this patch updates corresponding docs to reflect
> > Hugetlb entry meaning and difference between Hugetlb and
> > HugePages_Total * Hugepagesize.
> > 
> > ...
> >
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -2973,20 +2973,32 @@ int hugetlb_overcommit_handler(struct ctl_table *table, int write,
> >  
> >  void hugetlb_report_meminfo(struct seq_file *m)
> >  {
> > -	struct hstate *h = &default_hstate;
> > +	struct hstate *h;
> > +	unsigned long total = 0;
> > +
> >  	if (!hugepages_supported())
> >  		return;
> > -	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) {
> > +		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.

Thanks!