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. I don't know exactly what does that, but RDMA comes to mind. So does (ugh!) vmsplice, although I suspect that vmsplice doesn't write. --Andy -- 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>