Hi everybody, I'm trying to debug a video acquisition device driver and found myself having to dive deep into the memory management subsystem. The driver uses videobuf-dma-sg to manage video buffers. videobuf-dma-sg gets pointers to buffers from userspace and calls get_user_pages() to retrieve the list of pages underlying those buffers. The page list is used to build a scatter-gather list that is given to the hardware. The device then performs DMA directly to the memory. Pages underlying the buffers must obviously not be swapped out during DMA. The get_user_pages() (mm/memory.c) documentation seems to imply that returned pages are pinned to memory (my understanding of "pinned" is that they will not be swapped out): /** * get_user_pages() - pin user pages in memory * @tsk: task_struct of target task * @mm: mm_struct of target mm * @start: starting user address * @len: number of pages from start to pin * @write: whether pages will be written to by the caller * @force: whether to force write access even if user mapping is * readonly. This will result in the page being COWed even * in MAP_SHARED mappings. You do not want this. * @pages: array that receives pointers to the pages pinned. * Should be at least nr_pages long. Or NULL, if caller * only intends to ensure the pages are faulted in. * @vmas: array of pointers to vmas corresponding to each page. * Or NULL if the caller does not require them. However, all is seems to do for that purpose is incrementing the page reference count using get_page(). I had a look through the memory management subsystem code and it seems to me that incrementing the reference count is not sufficient to make sure the page won't be swapped out. To ensure that, it should instead be marked as unevictable, either directly or by marking an associated VMA as VM_LOCKED. This is what the mlock() syscall does, in addition to calling get_user_pages(). The MM subsystem is quite complex and my understanding might not be correct, so I'd appreciate if someone could shed light on the issue. Does get_user_pages() really pin pages to memory and prevent them from being swapped out in all circumstances ? If so, how does it do so ? If not, what's the proper way to make sure the pages won't disappear during DMA ? Please CC me on answers. Regards, Laurent Pinchart -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
