Re: m(un)map kmalloc buffers to userspace

From: Michal Hocko
Date: Wed Dec 09 2015 - 09:32:16 EST

On Wed 09-12-15 15:07:50, Marc Gonzalez wrote:
> 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);

So I guess you are mapping with 32pages granularity? If this is really
needed for internal usage you can use highorder page and map its
subpages directly.

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

I do not think there is any guarantee like that. AFAIK you only get
guarantee for the natural word alignment. Slab allocator is allowed
to use larger allocation and put its metadata or whatever before the
returned pointer.
Michal Hocko
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