Re: m(un)map kmalloc buffers to userspace

From: Michal Hocko
Date: Thu Dec 10 2015 - 09:06:38 EST

On Thu 10-12-15 14:37:38, Sebastian Frias wrote:
> On 12/10/2015 12:40 PM, Michal Hocko wrote:
> >On Wed 09-12-15 16:35:53, Sebastian Frias wrote:
> >[...]
> >>We've seen that drivers/media/pci/zoran/zoran_driver.c for example seems to
> >>be doing as us kmalloc+remap_pfn_range,
> >
> >This driver is broken - I will post a patch.
> Ok, we'll be glad to see a good example, please keep us posted.
> >
> >>is there any guarantee (or at least an advised heuristic) to determine
> >>if a driver is "current" (ie: uses the latest APIs and works)?
> >
> >OK, it seems I was overly optimistic when directing you to existing
> >drivers. Sorry about that I wasn't aware you could find such a terrible
> >code there. Please refer to Linux Device Drivers book which should give
> >you a much better lead (e.g.
> >
> Thank you for the link.
> The current code of our driver was has portions written following LDD3,
> however, we it seems that LDD3 advice is not relevant anymore.
> Indeed, it talks about VM_RESERVED, it talks about using "nopage" and it
> says that remap_pfn_range cannot be used for pages from get_user_page (or
> kmalloc).

Heh, it seems that we are indeed outdated there as well. The memory
management code doesn't really require pages to be reserved and it
allows to use get_user_page(s) memory to be mapped to user ptes.
remap_pfn_range will set all the appropriate flags to make sure MM code
will not stumble over those pages and let's the driver to take care of
the memory deallocation.

> It seems such assertions are valid on older kernels, because the code stops
> working on 3.4+ if we use remap_pfn_range the same way than
> drivers/media/pci/zoran/zoran_driver.c
> However, kmalloc+remap_pfn_range does work on 4.1.13+

As I've said nothing will guarantee that the kmalloc returned address
will be page aligned so you might corrupt slab internal data structures.
You might allocate a larger buffer via kmalloc and make sure it is
aligned properly but I fail to see why should be kmalloc used in the
first place as you need a memory in page size unnits anyway.
Michal Hocko
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