On Sun, Nov 20, 2011 at 01:39:12PM -0800, Hugh Dickins wrote: > > To be able to safely use mmap(), regarding SIGBUS, on files on the > > /dev/shm filesystem. The glibc fallback loop for -ENOSYS on fallocate > > is just ugly. > > The fallback for -EOPNOTSUPP? Probably for both. Note that the fallocate man page actually documents the errors incorrecly - it documents ENOSYS for filesystems not supporting fallocate, and EOPNOTSUPP for not recognizing the mode, but we actually return EOPNOTSUPP for either case. ENOSYS is only returned by kernels not implementing fallocate at all. > Being unfamiliar with glibc, I failed to find the internal_fallocate() > that it appears to use when the filesystem doesn't support the call; > so I don't know if I would agree with you that it's uglier than doing > the same(?) in the kernel. Last time I looked it basically did a pwrite loop writing zeroes. Unfortunately it did far too small I/O sizes and thus actually causes some major overhead e.g. on ext3. > But since the present situation is that tmpfs has one interface to > punching holes, madvise(MADV_REMOVE), that IBM were pushing 5 years ago; > but ext4 (and others) now a fallocate(FALLOC_FL_PUNCH_HOLE) interface > which IBM have been pushing this year: we do want to normalize that > situation and make them all behave the same way. FALLOC_FL_PUNCH_HOLE was added by Josef Bacik, who happens to work for Red Hat, but I doubt he was pushing any corporate agenda there, he was mostly making btrfs catch up with the 15 year old XFS hole punching ioctl. > And if tmpfs is going to support fallocate(FALLOC_FL_PUNCH_HOLE), > looking at Amerigo's much more attractive V2 patch, it would seem > to me perverse to permit the deallocation but fail the allocation. Agreed. -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>