On 04/10/2014 05:22 PM, David Herrmann wrote: > Hi > > On Thu, Apr 10, 2014 at 11:33 PM, Tony Battersby <[email protected]> > 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 from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
