Hi Sasha, Greg -

This is upstream commit 8e6e2ffa6569a205f1805cbaeca143b556581da6. I've received a request for it to be applied specifically to v5.15.y. Can you apply it to all LTS kernels back to v5.10 ?

-------- Forwarded Message -------- Subject: [PATCH] nfsd: add list_head nf_gc to struct nfsd_file Date: Wed, 10 Jul 2024 10:40:35 -0400 From: Youzhong Yang youzhong@gmail.com To: jlayton@kernel.org, chuck.lever@oracle.com, linux-nfs@vger.kernel.org, youzhong@gmail.com

nfsd_file_put() in one thread can race with another thread doing garbage collection (running nfsd_file_gc() -> list_lru_walk() -> nfsd_file_lru_cb()):

* In nfsd_file_put(), nf->nf_ref is 1, so it tries to do nfsd_file_lru_add(). * nfsd_file_lru_add() returns true (with NFSD_FILE_REFERENCED bit set) * garbage collector kicks in, nfsd_file_lru_cb() clears REFERENCED bit and returns LRU_ROTATE. * garbage collector kicks in again, nfsd_file_lru_cb() now decrements nf->nf_ref to 0, runs nfsd_file_unhash(), removes it from the LRU and adds to the dispose list [list_lru_isolate_move(lru, &nf->nf_lru, head)] * nfsd_file_put() detects NFSD_FILE_HASHED bit is cleared, so it tries to remove the 'nf' from the LRU [if (!nfsd_file_lru_remove(nf))]. The 'nf' has been added to the 'dispose' list by nfsd_file_lru_cb(), so nfsd_file_lru_remove(nf) simply treats it as part of the LRU and removes it, which leads to its removal from the 'dispose' list. * At this moment, 'nf' is unhashed with its nf_ref being 0, and not on the LRU. nfsd_file_put() continues its execution [if (refcount_dec_and_test(&nf->nf_ref))], as nf->nf_ref is already 0, nf->nf_ref is set to REFCOUNT_SATURATED, and the 'nf' gets no chance of being freed.

nfsd_file_put() can also race with nfsd_file_cond_queue(): * In nfsd_file_put(), nf->nf_ref is 1, so it tries to do nfsd_file_lru_add(). * nfsd_file_lru_add() sets REFERENCED bit and returns true. * Some userland application runs 'exportfs -f' or something like that, which triggers __nfsd_file_cache_purge() -> nfsd_file_cond_queue(). * In nfsd_file_cond_queue(), it runs [if (!nfsd_file_unhash(nf))], unhash is done successfully. * nfsd_file_cond_queue() runs [if (!nfsd_file_get(nf))], now nf->nf_ref goes to 2. * nfsd_file_cond_queue() runs [if (nfsd_file_lru_remove(nf))], it succeeds. * nfsd_file_cond_queue() runs [if (refcount_sub_and_test(decrement, &nf->nf_ref))] (with "decrement" being 2), so the nf->nf_ref goes to 0, the 'nf' is added to the dispose list [list_add(&nf->nf_lru, dispose)] * nfsd_file_put() detects NFSD_FILE_HASHED bit is cleared, so it tries to remove the 'nf' from the LRU [if (!nfsd_file_lru_remove(nf))], although the 'nf' is not in the LRU, but it is linked in the 'dispose' list, nfsd_file_lru_remove() simply treats it as part of the LRU and removes it. This leads to its removal from the 'dispose' list! * Now nf->ref is 0, unhashed. nfsd_file_put() continues its execution and set nf->nf_ref to REFCOUNT_SATURATED.

As shown in the above analysis, using nf_lru for both the LRU list and dispose list can cause the leaks. This patch adds a new list_head nf_gc in struct nfsd_file, and uses it for the dispose list. This does not fix the nfsd_file leaking issue completely.

Signed-off-by: Youzhong Yang youzhong@gmail.com --- fs/nfsd/filecache.c | 18 ++++++++++-------- fs/nfsd/filecache.h | 1 + 2 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c index ad9083ca144b..22ebd7fb8639 100644 --- a/fs/nfsd/filecache.c +++ b/fs/nfsd/filecache.c @@ -216,6 +216,7 @@ nfsd_file_alloc(struct net *net, struct inode *inode, unsigned char need, return NULL; INIT_LIST_HEAD(&nf->nf_lru); + INIT_LIST_HEAD(&nf->nf_gc); nf->nf_birthtime = ktime_get(); nf->nf_file = NULL; nf->nf_cred = get_current_cred(); @@ -393,8 +394,8 @@ nfsd_file_dispose_list(struct list_head *dispose) struct nfsd_file *nf; while (!list_empty(dispose)) { - nf = list_first_entry(dispose, struct nfsd_file, nf_lru); - list_del_init(&nf->nf_lru); + nf = list_first_entry(dispose, struct nfsd_file, nf_gc); + list_del_init(&nf->nf_gc); nfsd_file_free(nf); } } @@ -411,12 +412,12 @@ nfsd_file_dispose_list_delayed(struct list_head *dispose) { while(!list_empty(dispose)) { struct nfsd_file *nf = list_first_entry(dispose, - struct nfsd_file, nf_lru); + struct nfsd_file, nf_gc); struct nfsd_net *nn = net_generic(nf->nf_net, nfsd_net_id); struct nfsd_fcache_disposal *l = nn->fcache_disposal; spin_lock(&l->lock); - list_move_tail(&nf->nf_lru, &l->freeme); + list_move_tail(&nf->nf_gc, &l->freeme); spin_unlock(&l->lock); svc_wake_up(nn->nfsd_serv); } @@ -503,7 +504,8 @@ nfsd_file_lru_cb(struct list_head *item, struct list_lru_one *lru, /* Refcount went to zero. Unhash it and queue it to the dispose list */ nfsd_file_unhash(nf); - list_lru_isolate_move(lru, &nf->nf_lru, head); + list_lru_isolate(lru, &nf->nf_lru); + list_add(&nf->nf_gc, head); this_cpu_inc(nfsd_file_evictions); trace_nfsd_file_gc_disposed(nf); return LRU_REMOVED; @@ -578,7 +580,7 @@ nfsd_file_cond_queue(struct nfsd_file *nf, struct list_head *dispose) /* If refcount goes to 0, then put on the dispose list */ if (refcount_sub_and_test(decrement, &nf->nf_ref)) { - list_add(&nf->nf_lru, dispose); + list_add(&nf->nf_gc, dispose); trace_nfsd_file_closing(nf); } } @@ -654,8 +656,8 @@ nfsd_file_close_inode_sync(struct inode *inode) nfsd_file_queue_for_close(inode, &dispose); while (!list_empty(&dispose)) { - nf = list_first_entry(&dispose, struct nfsd_file, nf_lru); - list_del_init(&nf->nf_lru); + nf = list_first_entry(&dispose, struct nfsd_file, nf_gc); + list_del_init(&nf->nf_gc); nfsd_file_free(nf); } } diff --git a/fs/nfsd/filecache.h b/fs/nfsd/filecache.h index c61884def906..3fbec24eea6c 100644 --- a/fs/nfsd/filecache.h +++ b/fs/nfsd/filecache.h @@ -44,6 +44,7 @@ struct nfsd_file { struct nfsd_file_mark *nf_mark; struct list_head nf_lru; + struct list_head nf_gc; struct rcu_head nf_rcu; ktime_t nf_birthtime; };