Re: Do we really need d_weak_revalidate???

[NeilBrown <neilb@suse.com>]

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On Thu, Aug 17 2017, Ian Kent wrote:

> On 16/08/17 19:34, Jeff Layton wrote:
>> On Wed, 2017-08-16 at 12:43 +1000, NeilBrown wrote:
>>> 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.
>
> When this was first implemented autofs didn't use kern_path_mountpoint()
> (it didn't exist) it used a path lookup on the parent and a separate
> lookup for the last component.

This was before commit 4e44b6852e03 ("Get rid of path_lookup in
autofs4").  This used kern_path().

I'm more interested in commit ac8387199656 ("autofs4 - fix device ioctl
mount lookup")  which replaced the use of kern_path() with
kern_path_mountpoint().

>
> It's used for two operations, first to open a file handle to a (possibly)
> covered autofs mount, and second to get mounted information about a path
> without following past a (possibly covered) autofs mount.
>
> It's less about not triggering an automount or getting stuck in ->d_manage()
> and more about resolving paths that are not accessible via normal vfs walks.
>
> I never thought about re-validation for either of these cases and altering
> it to the way it was before filename_mountpoint() shouldn't be a
> problem.

If it shouldn't be a problem, what justified ac8387199656??

Thanks,
NeilBrown

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