linux-kernel - Re: [RFC] mm: Change the data type of huge page size from unsigned long to u64

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Message-Id: <09ab7941-07fa-0003-46d8-9fa5c07eba2d@linux.vnet.ibm.com>
Date:   Tue, 13 Sep 2016 19:21:07 +0800
From:   Rui Teng <rui.teng@...ux.vnet.ibm.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Chen Gang <chengang@...ndsoft.com.cn>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        hejianet@...ux.vnet.ibm.com
Subject: Re: [RFC] mm: Change the data type of huge page size from unsigned
 long to u64

On 9/13/16 5:32 PM, Kirill A. Shutemov wrote:
> On Tue, Sep 13, 2016 at 05:26:05PM +0800, Rui Teng wrote:
>> The huge page size could be 16G(0x400000000) on ppc64 architecture, and it will
>> cause an overflow on unsigned long data type(0xFFFFFFFF).
>
> Huh? ppc64 is 64-bit system and sizeof(void *) is equal to
> sizeof(unsigned long) on Linux (LP64 model).
>
> So where your 0xFFFFFFFF comes from?
>
The size of unsigned long data type is 4 bytes, and the 0xFFFFFFFF here
is the maximum value. And 16G is bigger than it.