linux-kernel - [PATCH] mm: fix negative commitlimit when gigantic hugepages are allocated

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Date:	Thu, 2 Jun 2011 23:55:57 -0300
From:	Rafael Aquini <aquini@...ux.com>
To:	Russ Anderson <rja@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux.com>, rja@...ricas.sgi.com
Subject: [PATCH] mm: fix negative commitlimit when gigantic hugepages are
 allocated

When 1GB hugepages are allocated on a system, free(1) reports
less available memory than what really is installed in the box.
Also, if the total size of hugepages allocated on a system is
over half of the total memory size, CommitLimit becomes
a negative number.

The problem is that gigantic hugepages (order > MAX_ORDER)
can only be allocated at boot with bootmem, thus its frames
are not accounted to 'totalram_pages'. However,  they are
accounted to hugetlb_total_pages()

What happens to turn CommitLimit into a negative number
is this calculation, in fs/proc/meminfo.c:

        allowed = ((totalram_pages - hugetlb_total_pages())
                * sysctl_overcommit_ratio / 100) + total_swap_pages;

A similar calculation occurs in __vm_enough_memory() in mm/mmap.c.

Also, every vm statistic which depends on 'totalram_pages' will render
confusing values, as if system were 'missing' some part of its memory.

Reported-by: Russ Anderson <rja@....com>
Signed-off-by: Rafael Aquini <aquini@...ux.com>
---
 mm/hugetlb.c |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index f33bb31..c67dd0f 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1111,6 +1111,14 @@ static void __init gather_bootmem_prealloc(void)
 		WARN_ON(page_count(page) != 1);
 		prep_compound_huge_page(page, h->order);
 		prep_new_huge_page(h, page, page_to_nid(page));
+
+		/* if we had gigantic hugepages allocated at boot time,
+		 * we need to reinstate the 'stolen' pages to totalram_pages,
+		 * in order to fix confusing memory reports from free(1)
+		 * and another side-effects, like CommitLimit going negative.
+		 */
+		if (h->order > (MAX_ORDER - 1))
+			totalram_pages += 1 << h->order;
 	}
 }

-- 
1.7.4.4

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