On Tue, 2023-09-19 at 13:10 -0700, Paul Eggert wrote: > On 2023-09-19 09:31, Jeff Layton wrote: > > The typical case for make > > timestamp comparisons is comparing source files vs. a build target. If > > those are being written nearly simultaneously, then that could be an > > issue, but is that a typical behavior? > > I vaguely remember running into problems with 'make' a while ago > (perhaps with a BSDish system) when filesystem timestamps were > arbitrarily truncated in some cases but not others. These files would > look older than they really were, so 'make' would think they were > up-to-date when they weren't, and 'make' would omit actions that it > should have done, thus screwing up the build. > > File timestamps can be close together with 'make -j' on fast hosts. > Sometimes a shell script (or 'make' itself) will run 'make', then modify > a file F, then immediately run 'make' again; the latter 'make' won't > work if F's timestamp is mistakenly older than targets that depend on it. > > Although 'make'-like apps are the biggest canaries in this coal mine, > the issue also affects 'find -newer' (as Bruno mentioned), 'rsync -u', > 'mv -u', 'tar -u', Emacs file-newer-than-file-p, and surely many other > places. For example, any app that creates a timestamp file, then backs > up all files newer than that file, would be at risk. > > > > I wonder if it would be feasible to just advance the coarse-grained > > current_time whenever we end up updating a ctime with a fine-grained > > timestamp? > > Wouldn't this need to be done globally, that is, not just on a per-file > or per-filesystem basis? If so, I don't see how we'd avoid locking > performance issues. > Maybe. Another idea might be to introduce a new timekeeper for multigrain filesystems, but all of those would likely have to share the same coarse-grained clock source. So yeah, if you stat an inode and then update it, any inode written on a multigrain filesystem within the same jiffy-sized window would have to log an extra transaction to write out the inode. That's what I meant when I was talking about write amplification. > > PS. Although I'm no expert in the Linux inode code I hope you don't mind > my asking a question about this part of inode_set_ctime_current: > > /* > * If we've recently updated with a fine-grained timestamp, > * then the coarse-grained one may still be earlier than the > * existing ctime. Just keep the existing value if so. > */ > ctime.tv_sec = inode->__i_ctime.tv_sec; > if (timespec64_compare(&ctime, &now) > 0) > return ctime; > > Suppose root used clock_settime to set the clock backwards. Won't this > code incorrectly refuse to update the file's timestamp afterwards? That > is, shouldn't the last line be "goto fine_grained;" rather than "return > ctime;", with the comment changed from "keep the existing value" to "use > a fine-grained value"? It is a problem, and Linus pointed that out yesterday, which is why I sent this earlier today: https://lore.kernel.org/linux-fsdevel/20230919-ctime-v1-1-97b3da92f504@xxxxxxxxxx/T/#u Bear in mind that we're not dealing with a situation where the value has not been queried since its last update, so we don't need to use a fine grained timestamp there (and really, it's preferable not to do so). A coarse one should be fine in this case. -- Jeff Layton <jlayton@xxxxxxxxxx>
