Return-Path: Received: from cantor2.suse.de ([195.135.220.15]:57716 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751578AbbGMDjp (ORCPT ); Sun, 12 Jul 2015 23:39:45 -0400 Date: Mon, 13 Jul 2015 13:39:34 +1000 From: NeilBrown To: Kinglong Mee Cc: Al Viro , "J. Bruce Fields" , "linux-nfs@vger.kernel.org" , linux-fsdevel@vger.kernel.org, Trond Myklebust Subject: Re: [PATCH 10/10 v7] nfsd: Allows user un-mounting filesystem where nfsd exports base on Message-ID: <20150713133934.6a4ef77d@noble> In-Reply-To: <55A111A8.2040701@gmail.com> References: <55A11010.6050005@gmail.com> <55A111A8.2040701@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: linux-nfs-owner@vger.kernel.org List-ID: On Sat, 11 Jul 2015 20:52:56 +0800 Kinglong Mee 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 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