On Thu, Aug 19, 2010 at 12:44:13PM +1000, Neil Brown wrote: > On Wed, 18 Aug 2010 22:08:03 -0400 > "J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote: > > > On Thu, Aug 19, 2010 at 10:52:18AM +1000, Neil Brown wrote: > > > On Thu, 19 Aug 2010 09:41:36 +1000 > > > Neil Brown <neilb@xxxxxxx> wrote: > > > > > > > So I agree that this is probably more of an issue for directories than for > > > > files, and that implementing it just for directories would be a sensible > > > > first step with lower expected overhead - just my reasoning seems to be a bit > > > > different. > > > > > > Just to be sure we are on the same page: > > > file_update_time would always refer to current_nfsd_time, but nfsd would > > > only update current_nfsd_time when a directory was examined (and the other > > > conditions were met). > > > > > > > > > So my current thinking on how this would look - names have been changed: > > > > > > - global timespec 'current_fs_precise_time' is zeroed when > > > current_kernel_time moves backwards and is protected by a seqlock > > > > > > - current_fs_time would be > > > now = max(current_kernel_time(), current_fs_precise_time) > > > return timespec_trunc(now, sb->s_time_gran) > > > (with appropriate seqlock protection) > > > > > > - new function in fs/inode.c > > > get_precise_time(timestamp) > > > > Odd name for something that returns nothing of interest; > > bump_precise_time() might be closer? > > > > And unique_time might be better than precise_time, since the property > > we're asking for is that mtime on a changed file by new? (Or > > versioned_time?) > > Agreed on both counts, tough I'm not keen on 'bump' myself. > got_unique_time() > because that it what we just did... I prefer the name to reflect why the > function is called, rather than what the function is expected to do about it. > never_use_this_timestamp_again(timestamp) > :-? Maybe "retire" for a pithier version of never_use_again: /** * retire_timestamp - prevent a timestamp from being reused as an mtime. * @timestamp * * Advance the clock used to generate mtimes to guarantee that the * given timestamp will not be reused on any future mtime update. * This allows the given timestamp to be passed back to users such as * nfs clients which need the guarantee that mtimes will always change * on file updates. * * Depending on the filesystem's s_time_gran this may not be an ironclad * guarantee. */ ? > > > > > > > cft = current_fs_time() > > > if (timestamp == cft) > > /* > > * Make sure the next mtime stored will be > > * something different from timestamp: > > */ > > > write_seqlock() > > > if cft == current_fs_precise_time > > > current_fs_precise_time.tv_nsec++ > > > else if cft > current_fs_precise_time > > > > What's the cft < current_fs_precise_time case? > > The current_fs_precise_time has been incremented with a resolution higher > than s_time_gran. i.e. s_time_gran > 1. > I'm not really sure what we want to do about that. > Maybe we should be incrementing tv_nsec by s_time_gran as long as that is > significantly less than jiffies_to_usec(1)*1000, but I don't know what I mean > by 'significantly'. How about just scratching "significantly" and saying "less"? As long as we know jiffies is the default time source for mtimes, that should be safe, shouldn't it? > The only values I can find for s_time_gran in current code are 1, 100, 1000 > and 1000000000. I didn't even know there were any other than 1 and a billion. OK! > All those are either way bigger than a jiffie or significantly smaller, but > suppose a filesystem came along that chose 1000000 (i.e. millisecond > timestamps) - should we increment tv_nsec by 1000000, or not, or cross that > bridge when we come to it? > > For reference: > default is 1000000000 (this would cover ext2, ext3, reiserfs, fat, sysv, ...) > cifs, smbfs, ntfs are 100 > udf, ceph are 1000 > rest (btrfs, ext4, gfs2, jfs, nilfs, ocfs2, xfs and virtual filesystems) are 1 Interesting list, thanks! --b. -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
