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Subject: Re: m(un)map kmalloc buffers to userspace
To: Michal Hocko <mhocko@kernel.org>
CC: Sebastian Frias <sebastian_frias@sigmadesigns.com>,
        linux-mm <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>
References: <5667128B.3080704@sigmadesigns.com>
 <20151209135544.GE30907@dhcp22.suse.cz>
From: Marc Gonzalez <marc_gonzalez@sigmadesigns.com>
Message-ID: <566835B6.9010605@sigmadesigns.com>
Date: Wed, 9 Dec 2015 15:07:50 +0100
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On 09/12/2015 14:55, Michal Hocko wrote:
> On Tue 08-12-15 18:25:31, Sebastian Frias wrote:
>> Hi,
>>
>> We are porting a driver from Linux 3.4.39+ to 4.1.13+, CPU is Cortex-A9.
>>
>> The driver maps kmalloc'ed memory to user space.
> 
> This sounds like a terrible idea to me. Why don't you simply use the
> page allocator directly? Try to imagine what would happen if you mmaped
> a kmalloc with a size which is not page aligned? mmaped memory uses
> whole page granularity.

According to the source code, this kernel module calls

  kmalloc(1 << 17, GFP_KERNEL | __GFP_REPEAT);

I suppose kmalloc() would return page-aligned memory?

(Note: the kernel module was originally written for 2.4 and was updated
inconsistently over the years.)

Regards.