Re: m(un)map kmalloc buffers to userspace

[Sebastian Frias]

On 12/10/2015 03:06 PM, Michal Hocko wrote:
> 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. http://www.makelinux.net/ldd3/chp-15-sect-2)
> >
> > 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 Heh, it seems that we are indeed outdated there as well.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.

Ok, just for information, do you know since when it is possible to use 
remap_pfn_range on kmalloc/get_user_page memory?

> > 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.

Ok, so let's say we stop using kmalloc in favor of __get_user_pages, do 
you see other things that would need to be done to be compliant with 
current practices?

For instance, drivers/media/pci/zoran/zoran_driver.c is doing:

   for (off = 0; off < fh->buffers.buffer_size; off += PAGE_SIZE)
      SetPageReserved(virt_to_page(mem + off));

on the memory allocated with kmalloc, but we are not doing any of that, 
yet it was working. Would the switch to __get_user_pages require the 
calls to SetPageReserved?

Thanks for your help.
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