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

NeilBrown
2015-07-13T03:39:45Z

On Sat, 11 Jul 2015 20:52:56 +0800 Kinglong Mee <kinglongmee@gmail.com>
wrote:

> If there are some mount points(not exported for nfs) under pseudo root,
> after client's operation of those entry under the root, anyone *can't*
> unmount those mount points until export cache expired.
>
> /nfs/xfs *(rw,insecure,no_subtree_check,no_root_squash)
> /nfs/pnfs *(rw,insecure,no_subtree_check,no_root_squash)
> total 0
> drwxr-xr-x. 3 root root 84 Apr 21 22:27 pnfs
> drwxr-xr-x. 3 root root 84 Apr 21 22:27 test
> drwxr-xr-x. 2 root root 6 Apr 20 22:01 xfs
> Filesystem 1K-blocks Used Available Use% Mounted on
> ......
> /dev/sdd 1038336 32944 1005392 4% /nfs/pnfs
> /dev/sdc 10475520 32928 10442592 1% /nfs/xfs
> /dev/sde 999320 1284 929224 1% /nfs/test
> /mnt/pnfs/:
> total 0
> -rw-r--r--. 1 root root 0 Apr 21 22:23 attr
> drwxr-xr-x. 2 root root 6 Apr 21 22:19 tmp
>
> /mnt/xfs/:
> total 0
> umount: /nfs/test/: target is busy
> (In some cases useful info about processes that
> use the device is found by lsof(8) or fuser(1).)
>
> It's caused by exports cache of nfsd holds the reference of
> the path (here is /nfs/test/), so, it can't be umounted.
>
> I don't think that's user expect, they want umount /nfs/test/.
> Bruce think user can also umount /nfs/pnfs/ and /nfs/xfs.
>
> Also, using kzalloc for all memory allocating without kmalloc.
> Thanks for Al Viro's commets for the logic of fs_pin.
>
> v3,
> 1. using path_get_pin/path_put_unpin for path pin
> 2. using kzalloc for memory allocating
>
> v4,
> 1. add a completion for pin_kill waiting the reference is decreased to zero.
> 2. add a work_struct for pin_kill decreases the reference indirectly.
> 3. free svc_export/svc_expkey in pin_kill, not svc_export_put/svc_expkey_put.
> 4. svc_export_put/svc_expkey_put go though pin_kill logic.
>
> v5, same as v4.
>
> v6,
> 1. Pin vfsmnt to mount point at first, when reference increace (==2),
> grab a reference to vfsmnt by mntget. When decreace (==1),
> drop the reference to vfsmnt, left pin.
> 2. Delete cache_head directly from cache_detail.
>
> v7,
> implement self reference increase and decrease for nfsd exports/expkey
>
> When reference of cahce_head increase(>1), grab a reference of mnt once.
> and reference decrease to 1 (==1), drop the reference of mnt.
>
> So after that,
> When ref > 1, user cannot umount the filesystem with -EBUSY.
> when ref ==1, means cache only reference by nfsd cache,
> no other reference. So user can try umount,
> 1. before set MNT_UMOUNT (protected by mount_lock), nfsd cache is
> referenced (ref > 1, legitimize_mntget), umount will fail with -EBUSY.
> 2. after set MNT_UMOUNT, nfsd cache is referenced (ref == 2),
> legitimize_mntget will fail, and set cache to CACHE_NEGATIVE,
> and the reference will be dropped, re-back to 1.
> So, pin_kill can delete the cache and umount success.
> 3. when umountting, no reference to nfsd cache,
> pin_kill can delete the cache and umount success.
>
> Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>

Wow.... this is turning out to be a lot more complex that I imagined at
first (isn't that always the way!).

There is a lot of good stuff here, but I think we can probably make it
simpler and so even better.

I particularly don't like the get_ref/put_ref pointers in cache_head.
They make cache_head a lot bigger than it was before, and they are only
needed for two specific caches. And then they are the same for every element
in the cache.

I also don't like the ref_mutex ... or I don't like where it is used...
or something. I definitely don't think we need one per cached entry.
Maybe one per cache.

I can certainly see that the "first" time we get a reference to a cache
item that holds a vfsmnt pointer, we need to "legitimize" that - or
fail. But I don't think that has to happen inside the cache.c
machinery.

How about this:
- add a new cache flag "CACHE_ACTIVE" (for example) which the cache
owner can set whenever it likes. When cache_put finds that CACHE_ACTIVE
is set when refcount is <= 2, it calls a new cache_detail method: cache_deactivate.
- cache_deactivate takes a mutex (yes, we do need one, don't we)
and if CACHE_ACTIVE is still set and refcount is still <= 2,
it drops the reference on the vfsmnt and clears CACHE_ACTIVE.
This actually needs to be something like:
if (test_and_clear_bit(CACHE_ACTIVE,...)) {
if (atomic_read(..refcnt) > 2) {
set_bit(CACHE_ACTIVE);
mutex_unlock()
return

so that if other code gets a reference and tests CACHE_ACTIVE, it
won't suddenly become inactive. Might need a memory barrier in there...
no, test_and_clear implies a memory barrier.

We only need to make changes to svc_export and svc_expkey - right?
So that would be:
Change svc_export_lookup and svc_expkey_lookup so they look something
like:

svc_XX_lookup(struct cache_detail *cd, struct svc_XXX *item)
{
struct cache_head *ch;
int hash = svc_XXX_hash(item);

ch = sunrpc_cache_lookup(cd, &item->h, hash);
if (!ch)
return NULL;
item = container_of(ch, struct svc_XXX, h);
if (!test_bit(CACHE_VALID, &ch->flags) ||
test_bit(CACHE_NEGATIVE, &ch->flags) ||
test_bit(CACHE_ACTIVE, &ch->flags))
return item;

mutex_lock(&svc_XXX_mutex);
if (!test_bit(CACHE_ACTIVE, &ch->flags)) {
if (legitimize_mnt_get() == NULL) {
XXX_put(item);
item = NULL;
} else
set_bit(CACHE_ACTIVE, &ch->flags);
}
mutex_unlock(&something);
return item;
}

Then the new 'cache_deactivate' function is something like:

svc_XXX_deactivate(struct cache_detail *cd, struct cache_head *ch)
{
struct svc_XXX *item = container_of(ch, &item->h, item);

mutex_lock(&svc_XXX_mutex);
if (test_and_clear_bit(CACHE_ACTIVE, &ch->flags)) {
if (atomic_read(&ch->ref.refcount) > 2) {
/* Race with get_ref - do nothing */
set_bit(CACHE_ACTIVE, &ch->flags);
else
mntput(....mnt);
}
mutex_unlock(&svc_XXX_mutex);
}

cache_put would have:

if (test_bit(CACHE_ACTIVE, &h->flags) &&
cd->cache_deactivate &&
atomic_read(&h->ref.refcount <= 2))
cd->cache_deactivate(cd, h);

but there is still a race. If: (T1 and T2 are threads)
T1: cache_put finds refcount is 2 and CACHE_ACTIVE is set and calls ->cache_deactiveate
T2: cache_get increments the refcount to 3
T1: cache_deactivate clears CACHE_ACTIVE and find refcount is 3
T2: now calls cache_put, which sees CACHE_ACTIVE is clear so refcount becomes 2
T1: sets CACHE_ACTIVE again and continues. refcount becomes 1.

So not refcount is 1 and the item is still active.

We can fix this by making cache_put loop:
while (test_bit(CACHE_ACTIVE, &h->flags) &&
cd->cache_deactivate &&
(smb_rmb(), 1) &&
atomic_read(&h->ref.refcount <= 2))
cd->cache_deactivate(cd, h);

This should ensure that refcount never gets to 1 with the
item still active (i.e. with a ref count on the mnt).

The work item and completion are a bit unfortunate too.

I guess the problem here is that pin_kill() can run while there are
some inactive references to the cache item. There can then be a race
over who will use path_put_unpin to put the dentry.

Could we fix that by having expXXX_pin_kill() use kref_get_unless_zero()
on the cache item.
If that succeeds, then path_put_unpin hasn't been called and it won't be.
So expXXX_pin_kill can call it and then set CACHE_NEGATIVE.
If it fails, then it has already been called and nothing else need be done.
Almost.
If kref_get_unless_zero() fails, pin_remove() may not have been called
yet, but it will be soon. We might need to wait.
It would be nice if pin_kill() would check ->done again after calling p->kill.
e.g.

diff --git a/fs/fs_pin.c b/fs/fs_pin.c
index 611b5408f6ec..c2ef5c9d4c0d 100644
--- a/fs/fs_pin.c
+++ b/fs/fs_pin.c
@@ -47,7 +47,9 @@ void pin_kill(struct fs_pin *p)
spin_unlock_irq(&p->wait.lock);
rcu_read_unlock();
p->kill(p);
- return;
+ if (p->done > 0)
+ return;
+ spin_lock_irq(&p->wait.lock);
}
if (p->done > 0) {
spin_unlock_irq(&p->wait.lock);

I think that would close the last gap, without needing extra work
items and completion in the nfsd code.

Al: would you be OK with that change to pin_kill?

Thanks,
NeilBrown