Re: Do we really need d_weak_revalidate???

[Ian Kent <ikent@redhat.com>]

On 21/08/17 14:23, NeilBrown wrote:
> On Fri, Aug 18 2017, Ian Kent wrote:
> 
>> On 18/08/17 13:24, NeilBrown wrote:
>>> 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 have to plead not guilty of this one.
>>
>> IIRC it broke the requirement of "lookup parent then lookup last component"
>> rather it walked the whole path then followed up to find the mount point
>> struct path.
>>
>> Like it says in the description of ac8387199656 the caller might not yet
>> "own" the autofs mount which causes a mount to be triggered during the
>> walk that can't be satisfied because of the deadlock that occurs.
> 
> A mount isn't triggered by kern_path(pathname, 0, &path).
> That '0' would need to include one of
>   LOOKUP_PARENT | LOOKUP_DIRECTORY |
>   LOOKUP_OPEN | LOOKUP_CREATE | LOOKUP_AUTOMOUNT
> 
> to trigger an automount (otherwise you just get -EISDIR).

It's perfectly sensible to think that but there is a case where a
a mount is triggered when using kern_path().

The EISDIR return occurs for positive dentrys, negative dentrys
will still trigger an automount (which is autofs specific,
indirect mount map using nobrowse option, the install default).

> 
> That is why I assumed that ->d_managed was the problem.
> 
>>
>>>
>>> I'm more interested in commit ac8387199656 ("autofs4 - fix device ioctl
>>> mount lookup")  which replaced the use of kern_path() with
>>> kern_path_mountpoint().
>>
>> Probably should have had a Fixes: 4e44b6852e03 ...
> 
> Still a bit confused as to exactly what was fixed...

Hopefully also considering the negative dentry case will clear that up.

Ian