Hi Al, >From a long time, I have been facing two issues in Linux kernel related to Direct I/O and Asynchronous I/O. Although so far I have managed to work around them, but I feel these could be easily fixed with a kernel patch. But before working out on a patch, I thought it is worthy to ask for an opinion about these two issues: 1. Direct I/O on Kernel Buffers: Almost all filesystems supporting Direct I/O lock down user space pages in memory for DMA (or network transfer) using get_user_pages (or equivalent) function. This function strictly assumes pages to be locked down are in user process' (calling process) address space and hence looks for pages in all virtual memory areas (vm_area_struct) holding the required user space address range, finds its page mapping in page tables and locks them in memory. This function returns -EINVAL for kernel buffers. 2. Asynchronous I/O Completion: AIO framework maintains an IO completion event ring buffer in address space of the requester process. When a filesystem completes AIO request (kiocb), it calls aio_complete function which puts a completion event in AIO ring buffer and application can consider AIO to be done. There is no way to track down completion. Above two limitation do not allow Direct I/O or asynchronous I/O to be issued from kernel, using kernel buffers. As a use-case, stackable filesystems like ecryptfs can manipulate (encrypt/decrypt) kernel buffers and support direct I/O + Asynchronous I/O. A database engine heavily rely on this combination and encryption has a better use case there. It is possible that these issues are already known and have not been fixed for a good reason, but would be great to know anyways. Thanks, Rajat -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
