On Mon, Aug 14 2017, Jeff Layton wrote: > On Mon, 2017-08-14 at 09:36 +1000, NeilBrown wrote: >> On Fri, Aug 11 2017, Jeff Layton wrote: >> >> > On Fri, 2017-08-11 at 05:55 +0000, Trond Myklebust wrote: >> > > On Fri, 2017-08-11 at 14:31 +1000, NeilBrown wrote: >> > > > Funny story. 4.5 years ago we discarded the FS_REVAL_DOT superblock >> > > > flag and introduced the d_weak_revalidate dentry operation instead. >> > > > We duly removed the flag from NFS superblocks and NFSv4 superblocks, >> > > > and added the new dentry operation to NFS dentries .... but not to >> > > > NFSv4 >> > > > dentries. >> > > > >> > > > And nobody noticed. >> > > > >> > > > Until today. >> > > > >> > > > A customer reports a situation where mount(....,MS_REMOUNT,..) on an >> > > > NFS >> > > > filesystem hangs because the network has been deconfigured. This >> > > > makes >> > > > perfect sense and I suggested a code change to fix the problem. >> > > > However when a colleague was trying to reproduce the problem to >> > > > validate >> > > > the fix, he couldn't. Then nor could I. >> > > > >> > > > The problem is trivially reproducible with NFSv3, and not at all with >> > > > NFSv4. The reason is the missing d_weak_revalidate. >> > > > >> > > > We could simply add d_weak_revalidate for NFSv4, but given that it >> > > > has been missing for 4.5 years, and the only time anyone noticed was >> > > > when the ommission resulted in a better user experience, I do wonder >> > > > if >> > > > we need to. Can we just discard d_weak_revalidate? What purpose >> > > > does >> > > > it serve? I couldn't find one. >> > > > >> > > > Thanks, >> > > > NeilBrown >> > > > >> > > > For reference, see >> > > > Commit: ecf3d1f1aa74 ("vfs: kill FS_REVAL_DOT by adding a >> > > > d_weak_revalidate dentry op") >> > > > >> > > > >> > > > >> > > > To reproduce the problem at home, on a system that uses systemd: >> > > > 1/ place (or find) a filesystem image in a file on an NFS filesystem. >> > > > 2/ mount the nfs filesystem with "noac" - choose v3 or v4 >> > > > 3/ loop-mount the filesystem image read-only somewhere >> > > > 4/ reboot >> > > > >> > > > If you choose v4, the reboot will succeed, possibly after a 90second >> > > > timeout. >> > > > If you choose v3, the reboot will hang indefinitely in systemd- >> > > > shutdown while >> > > > remounting the nfs filesystem read-only. >> > > > >> > > > If you don't use "noac" it can still hang, but only if something >> > > > slows >> > > > down the reboot enough that attributes have timed out by the time >> > > > that >> > > > systemd-shutdown runs. This happens for our customer. >> > > > >> > > > If the loop-mounted filesystem is not read-only, you get other >> > > > problems. >> > > > >> > > > We really want systemd to figure out that the loop-mount needs to be >> > > > unmounted first. I have ideas concerning that, but it is messy. But >> > > > that isn't the only bug here. >> > > >> > > The main purpose of d_weak_revalidate() was to catch the issues that >> > > arise when someone changes the contents of the current working >> > > directory or its parent on the server. Since '.' and '..' are treated >> > > specially in the lookup code, they would not be revalidated without >> > > special treatment. That leads to issues when looking up files as >> > > ./<filename> or ../<filename>, since the client won't detect that its >> > > dcache is stale until it tries to use the cached dentry+inode. >> > > >> > > The one thing that has changed since its introduction is, I believe, >> > > the ESTALE handling in the VFS layer. That might fix a lot of the >> > > dcache lookup bugs that were previously handled by d_weak_revalidate(). >> > > I haven't done an audit to figure out if it actually can handle all of >> > > them. >> > > >> > >> > It may also be related to 8033426e6bdb2690d302872ac1e1fadaec1a5581: >> > >> > vfs: allow umount to handle mountpoints without revalidating them >> >> You say in the comment for that commit: >> >> but there >> are cases where we do want to revalidate the root of the fs. >> >> Do you happen to remember what those cases are? >> > > Not exactly, but I _think_ I might have been assuming that we needed to > ensure that the inode attrs on the root were up to date after the > pathwalk. > > I think that was probably wrong. d_revalidate is really intended to > ensure that the dentry in question still points to the same inode. In > the case of the root of the mount though, we don't really care about the > dentry on the server at all. We're attaching the root of the mount to an > inode and don't care of the dentry name changes. If we do need to ensure > the inode attrs are updated, we'll just revalidate them at that point. > >> > >> > Possibly the fact that we no longer try to revalidate during unmount >> > means that this is no longer necessary? >> > >> > The original patch that added d_weak_revalidate had a reproducer in the >> > patch description. Have you verified that that problem is still not >> > reproducible when you remove d_weak_revalidate? >> >> I did try the reproducer and it works as expected both with and without >> d_weak_revalidate. >> On reflection, the problem it displayed was caused by d_revalidate() >> being called when the dentry name was irrelevant. We remove that >> (fixing the problem) and introduce d_weak_revalidate because we thought >> that minimum functionality was still useful. I'm currently not >> convinced that even that is needed. >> >> If we discarded d_weak_revalidate(), we could get rid of the special >> handling of umount.... > > I like idea. I say go for it and we can see what (if anything) breaks?
Getting rid of d_weak_revalidate is easy enough - hardly any users. Getting rid of filename_mountpoint() isn't so easy unfortunately. autofs4 uses kern_path_mountpoint() - presumably to avoid getting stuck in autofs4_d_manage()? It would be a shame to keep this infrastructure around just so that one part of autofs4 can talk to another part of autofs4. Do you know if the fact that filename_mountpoint() skips ->d_manage is important for sys_umount ?? Thanks, NeilBrown > -- > Jeff Layton <jlay...@redhat.com>
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