On Fri, Jul 19, 2013 at 12:19:30PM -0400, Theodore Ts'o wrote: > On Fri, Jul 19, 2013 at 01:33:09PM +1000, Dave Chinner wrote: > > An ioctl is kinda silly for this. Just use O_NONBLOCK when calling > > open() and do the prefetch right in the open call. The open() can > > block, anyway, and what you are trying to do is non-blocking IO with > > AIO, so it seems like we've already got a sensible, generic > > interface for triggering this sort of prefetch operation. > > O_NONBLOCK (either set via open or fcntl) is a possibility, since it's > carefully defined to be unspecified for regular files by SUSv3. It is > quite different from the existing semantics for O_NONBLOCK, though. > Currently, for all file types where O_NONBLOCK is not ignored, open(2) > is guaranteed itself not to block. If we use O_NONBLOCK for regular > files to mean that any necessary metadata blocks required for AIO to > be "A" will be cached, then it will make open(2) much more likely to > block. Also, for all file types where O_NONBLOCK is not ignored, > read(2) will not block but instead return -1 and set errno to EAGAIN. > This would also be a change. > > If we tried to get this new semantics for O_NONBLOCK to be accepted by > the Austin Group for standardization in the future, would they accept > it, or would they say, "this makes me vommit"? I have a suspicion > there reaction might be closer to the latter.... > > If we want a VFS-level API, in my opinion an fadvise() flag would be a > better choice. Sure. Make it an fadvise() flag - just don't add ioctls for things that are generically useful. On second thoughts - you're trying to get the extent map read in. We already have an interface for querying extent maps - fiemap. FIEMAP_FLAG_PREFETCH along with the range of the file you want the extent map prefetched for? Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
