On Tue, Mar 19, 2013 at 6:03 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: > On Tue, Mar 19, 2013 at 11:29:41AM +0100, Miklos Szeredi wrote: > >> Copy up is a once-in-a-lifetime event for an object. Optimizing it is >> way down in the list of things to do. I'd drop splice in a jiffy if >> it's in the way. > > What makes you think that write is any better? Same deadlock there - check > generic_file_aio_write(), it calls the same sb_start_write()... IOW, > switching from splice to write won't help at all. Okay, I missed that. Yeah, that needs fixing... >> Much more interesting question: what happens if we crash during a >> rename? Whiteout implemented in the filesystem won't save us. And >> the results are interesting: old versions of files become visible and >> similar fun. Far from likely to happen, but ... >> >> Add a rename-with-whiteout primitive on filesystems? That one is not >> going to be as simple as plain whiteout. Or? > > Umm... If/when we start caring about that kind of atomicity (and I agree > that we ought to) overlayfs approach to whiteouts will actually have much > harder time - it doesn't take much to teach a journalling fs how to do that > kind of ->rename() in a single transaction; the only question is how to tell > it that we want to leave a whiteout behind us. Hell knows; one variant is > to add a flag, of course. Another might be more interesting - we want some > kind of "directory is opaque" flag, so if we start reshuffling the methods, > we might try to merge unlink/rmdir/whiteout. Rules: > * victim is negative => create a whiteout > * victim is a directory, parent opaque => rmdir > * victim is a non-directory, parent opaque => unlink > * victim is positive, parent _not_ opaque => replace with whiteout > * old_dir in case of ->rename() is opaque => normal rename > * old_dir in case of ->rename() is not opaque => leave whiteout behind > Non-unioned => opaque, of course (nothing showing through it). > I dunnow. Overloading common paths with overlay/union specific things doesn't look very clean to me. I have a similar problem with union-mounts: it's hooking into lots of common paths in the VFS for the sake of a very specialized feature. > Getting good behaviour on rename interrupted by crash is going to be _very_ > tricky with any strategy other than whiteouts-in-fs, AFAICS. > One idea is to add a journal to the overlay itself (yeah, namespace issues). Thanks, Miklos -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
