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

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Date:	Thu, 9 Jun 2011 16:44:08 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	aquini@...ux.com
Cc:	Russ Anderson <rja@....com>,
	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: Re: [PATCH] mm: fix negative commitlimit when gigantic hugepages
 are allocated

On Thu, 2 Jun 2011 23:55:57 -0300
Rafael Aquini <aquini@...ux.com> wrote:

> 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.

Is this bug serious enough to justify backporting the fix into -stable
kernels?

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