On Tue, Sep 08, 2015 at 04:08:43PM -0700, Andy Lutomirski wrote: > Can we have a clean way to figure out whether two file ranges are the > same in a way that allows false negatives? I.e. return 1 if the > ranges are reflinks of each other and 0 if not? Pretty please? I've > implemented that in the past on btrfs by syncing the ranges and then > comparing FIEMAP output, but that's hideous. That fundamentally doesn't work for userspace, because the moment the filesystem drops it's locks on the inodes in the kernel after doing the comparison the mappings can change. IOWs, by the time the information gets back to userspace, it's already wrong. e.g. cp made this mistake by trying to use FIEMAP to optimise hole detection in files and ended up with corrupt copies. It really doesn't matter what the syscall/ioctl interface is, trying to make application logic decisions based on inode block mappings from userspace is racy and not safe and will go wrong... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
