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 Heh, it seems that we are indeed outdated there as well.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. > 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 SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>