Re: [PATCH 10/10 v7] nfsd: Allows user un-mounting filesystem where nfsd exports base on

[Kinglong Mee <kinglongmee@gmail.com>]

On 7/24/2015 10:05, NeilBrown wrote:
> On Mon, 13 Jul 2015 05:45:53 +0100 Al Viro <viro@ZenIV.linux.org.uk>
> wrote:
> 
>> On Mon, Jul 13, 2015 at 02:20:59PM +1000, NeilBrown wrote:
>>
>>> Actually, with that change to pin_kill, this side of things becomes
>>> really easy.
>>> All expXXX_pin_kill needs to do is call your new cache_delete_entry.
>>> If that doesn't cause the entry to be put, then something else has a
>>> temporary reference which will be put soon.  In any case, pin_kill()
>>> will wait long enough, but not indefinitely.
>>> No need for kref_get_unless_zero() or any of that.
>>
>> No.  You are seriously misunderstanding what ->kill() is for and what the
>> existing instances are doing.  Again, there is no promise whatsoever that
>> the object containing fs_pin instance will *survive* past ->kill().
>> At all.
>>
>> RTFS, please.  What is sorely missing in this recurring patchset is a clear
>> description of lifetime rules and ordering (who waits for whom and how long).
>> For all the objects involved.
> 
> Good point.  Let me try.
> 
> Entries in the sunrpc 'cache' each contain some 'key' fields and some
> 'content' fields.
> 
> The key fields are set by the .init() method when the entry is
> created, which can happen in a call to sunrpc_cache_lookup() or to
> sunrpc_cache_update().
> 
> The content fields are set by the .update() method when a value is
> provided for the cache entry.  This happens in sunrpc_cache_update();
> 
> A cache entry can be not-valid, negative, or valid.
> It starts non-valid when sunrpc_cache_lookup() fails to find the search
> key and so creates a new entry (and sets up the key with .init).
> It then transitions to either negative or valid.
> This can happen through sunrpc_cache_update() or through an error when
> instigating an up-call, in which case it goes to negative.
> Once it is negative or valid, it stays that way until it is released.
> If sunrpc_cache_update is called on an entry that is not not-valid,
> then a new entry is created and the old one is marked as expired.
> A cache search will find the new one before the old.
> 
> The vfsmount object is involved in two separate caches.
> It is part of the content of svc_expkey and part of the key of
> svc_export.
> 
> An svc_expkey entry is only ever held transiently.  It is held while an
> update is being processed, and it is held briefly while mapping a
> filehandle to a mnt+dentry.
> Firstly part of the filehandle is used to acccess the svc_expkey cache
> to get the vfsmnt.  Then that vfsmnt plus the client identification is
> looked up in the svc_export cache to find the export options.  Then the
> svc_expkey cache entry is released.
> 
> So it is only held during a lookup of another cache.  This can take an
> arbitrarily long time as the lookup can go to rpc.mountd in user-space.
> 
> 
> The svc_export cache entry can be held for the duration of a single NFS
> request.  It is stored in the 'struct svc_fh' file handle structure
> which is release at the end of handling the request.
> 
> The vfsmnt and dentry are only "used" to validate the filehandle and
> then while that filehandle is still active.
> 
> 
> To avoid having unmount hang while nfsd is performing an upcall to
> mountd, we need to legitimize the vfsmnt in the svc_expkey.  If that
> fails, exp_find_key() can fail and we would never perform the lookup on
> svc_export.
> 
> If it succeeds, then the legitimacy can be handed over to the svc_export
> cache entry, which could then continue to own it, or could hand it on
> to the svc_fh.
> 
> The latter is *probably* cleanest.
> i.e. an svc_fh should always own a reference to exp->ex_path.mnt, and
> fh_put must put it.

I don't agree adding new argument (eg, fh_vfsmnt) in svc_fh.

With it, should nfsd using fh_vfsmnt always, never using exp->ex_path.mnt
outside of export.c/export.h ?

If choose fh_vfsmnt, so many codes need be updated, especially functions.
If exp->ex_path.mnt, the new argument fh_vfsmnt seems redundant.

Thanks for your work.

It reminders a new method,

1. There are only one outlet from each cache, exp_find_key() for expkey, 
   exp_get_by_name() for export.
2. Any fsid to export or filehandle to export will call the function.
3. exp_get()/exp_put() increase/decrease the reference of export.

Like the fh_vfsmnt (not same), call legitimize_mntget() in the only
outlet function exp_find_key()/exp_get_by_name(), if fail return STALE,
otherwise, any valid expkey/export from the cache is validated (Have
get the reference of vfsmnt).

Add mntget() in exp_get() and mntput() in exp_put(), because the export
passed to exp_get/exp_put are returned from exp_find_key/exp_get_by_name.

> 
> exp_find_key needs to legitimize ek->ek_path.mnt, so a successful
> return from exp_find implies an active refernece to ->ex_path.mnt.
> If exp_find fails, it needs to mnt_put(ek->ek_path.mnt).

Yes, it's great.

> All callers of exp_find need to mnt_put(exp->ex_path.mnt) when they
> decide not to use the exp, and must otherwise store it in an svc_fh.
> 
> With this, pin_kill() should only need to wait for  exp_find_key() to
> discover that it cannot legitimize the mount, or for expkey_path() to
> replace the key via sunrpc_cache_update(), or maybe for cache_clean()
> to discard an old entry.
> 
> Hopefully that makes it all clear.

Yes, thanks again.

With my method, for expkey cache,
1. At first, a fsid is passed to exp_find_key, and lookup a cache
   in svc_expkey_lookup, if success, ekey->ek_path is pined to mount.
2. Then call legitimize_mntget getting a reference of vfsmnt 
   before return from exp_find_key.
3. Any calling exp_find_key with valid cache must put the vfsmnt.

for export cache,
1. At first, a path (returned from exp_find_key) with validate vfsmnt
   is passed to exp_get_by_name, if success, exp->ex_path is pined to mount.
2. Then call legitimize_mntget getting a reference of vfsmnt 
   before return from exp_get_by_name.
3. Any calling exp_get_by_name with valid cache must put the vfsmnt
   by exp_put();
4. Any using the exp returned from exp_get_by_name must call exp_get(),
   will increase the reference of vfsmnt.

So,
a. After getting the reference in 2, any umount of filesystem will get -EBUSY.
b. After put all reference after 4, or before get the reference in 2, 
   any umount of filesystem will call pin_kill, and delete the cache directly,
   also unpin the vfsmount.
c. Between 1 and 2, have get the reference of exp/key cache, with invalidate vfsmnt.
   As you said, umount of filesystem only wait exp_find_key/exp_get_by_name
   put the reference of cache when legitimize_mntget fail.

A new update of this patch site will be push later.

thanks,
Kinglong Mee