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Heh, it seems that we are indeed outdated there as well.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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