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Date: Wed, 30 Nov 2016 13:33:58 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Yisheng Xie <xieyisheng1@huawei.com>
Cc: <linux-kernel@vger.kernel.org>, <acme@redhat.com>,
        <mgorman@techsingularity.net>, <viro@zeniv.linux.org.uk>,
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        <arnd@arndb.de>, <guohanjun@huawei.com>, <qiuxishi@huawei.com>
Subject: Re: [PATCH] kernel/sysctl: return -EINVAL if write invalid val to
 ulong type sysctl
Message-Id: <20161130133358.d508cc8931be6f08ee88561f@linux-foundation.org>
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On Wed, 30 Nov 2016 18:30:52 +0800 Yisheng Xie <xieyisheng1@huawei.com> wrote:

> I tried to echo an invalid value to an unsigned long type sysctl on
> 4.9.0-rc6:
>    linux:~# cat /proc/sys/vm/user_reserve_kbytes
>    131072
>    linux:~# echo -1 > /proc/sys/vm/user_reserve_kbytes
>    linux:~# cat /proc/sys/vm/user_reserve_kbytes
>    131072
> 
> The echo operation got error and the value do not write to
> user_reserve_kbytes, however, user do not know it until checking
> the value again.
> 
> This patch return -EINVAL when write an invalid value to unsigned
> long type sysctl to make user know  what happened without
> checking its value once more, just as what proc_douintvec do.

hmpf.

# echo 18446744073709551615  > /proc/sys/vm/user_reserve_kbytes                                             
# cat /proc/sys/vm/user_reserve_kbytes
18446744073709551615

I think that when taking in an unsigned long the kernel should simply
treat -1 as 0xffffffff (or 0xffffffffffffffff).  It's natural and
normal and everyone knows what it means?