Andy Lutomirski wrote: > On 04/10/2014 05:22 PM, David Herrmann wrote: > >> Hi >> >> On Thu, Apr 10, 2014 at 11:33 PM, Tony Battersby <tonyb@xxxxxxxxxxxxxxx> wrote: >> >>> For O_DIRECT the kernel pins the submitted pages in memory for DMA by >>> incrementing the page reference counts when the I/O is submitted, >>> allowing the pages to be modified by DMA even if they are no longer >>> mapped in the address space of the process. This is different from a >>> regular read(), which uses the CPU to copy the data and will fail if the >>> pages are not mapped. >>> >> Can you please provide an example code-path? For instance, >> file_read_actor() does not pin any pages but only keeps the user-space >> address and resolves it once it has data to write. >> > > This may be an issue for anything in the kernel that calls > get_user_pages and holds onto the result at any time that mmap_sem isn't > held. > > Exactly. For O_DIRECT, that would be the call to get_user_pages_fast() from dio_refill_pages() in fs/direct-io.c, which is ultimately called from blkdev_direct_IO(). >From the comment for get_user_pages_fast(): "Attempt to pin user pages in memory..." Tony -- 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>