this feature could be used to memory region allocated by malloc() in user mode and mmaped memory region allocated by other memory allocators. userptr interface can identify memory type through vm_flags value and would get pages or page frame numbers to user space appropriately.
Changelog v4: we have discussed some issues to userptr feature with drm and mm guys and below are the issues. The migration issue. - Pages reserved by CMA for some device using DMA could be used by kernel and if the device driver wants to use those pages while being used by kernel then the pages are copied into other ones allocated to migrate them and then finally, the device driver can use the pages for itself. Thus, migrated, the pages being accessed by DMA could be changed to other so this situation may incur that DMA accesses any pages it doesn't want. The COW issue. - while DMA of a device is using the pages to VMAs, if current process was forked then the pages being accessed by the DMA would be copied into child's pages.(Copy On Write) so these pages may not have coherrency with parent's ones if child process wrote something on those pages so we need to flag VM_DONTCOPY to prevent pages from being COWed. But the use of get_user_pages is safe from such magration issue because all the pages from get_user_pages CAN NOT BE not only migrated but also swapped out. this true has been confirmed by mm guys, Minchan Kim and KOSAKI Motohiro. However below issue could be incurred. The deterioration issue of system performance by malicious process. - any malicious process can request all the pages of entire system memory through this userptr ioctl and as the result, all other processes would be blocked and this would incur the deterioration of system performance because the pages couldn't be swapped out. So we limit user-desired userptr size to pre-defined and this ioctl command CAN BE accessed by only root user. Changelog v3: Mitigated the issues pointed out by Dave and Jerome. for this, added some codes to guarantee the pages to user space not to be swapped out, the VMAs within the user space would be locked and then unlocked when the pages are released. but this lock might result in significant degradation of system performance because the pages couldn't be swapped out so added one more featrue that we can limit user-desired userptr size to pre-defined value using userptr limit ioctl that can be accessed by only root user. Changelog v2: the memory regino mmaped with VM_PFNMAP type is physically continuous and start address of the memory region should be set into buf->dma_addr but previous patch had a problem that end address is set into buf->dma_addr so v2 fixes that problem. Inki Dae (2): drm/exynos: added userptr limit ioctl. drm/exynos: added userptr feature. drivers/gpu/drm/exynos/exynos_drm_drv.c | 8 + drivers/gpu/drm/exynos/exynos_drm_drv.h | 6 + drivers/gpu/drm/exynos/exynos_drm_gem.c | 413 +++++++++++++++++++++++++++++++ drivers/gpu/drm/exynos/exynos_drm_gem.h | 20 ++- include/drm/exynos_drm.h | 43 +++- 5 files changed, 487 insertions(+), 3 deletions(-) -- 1.7.4.1
