nfsd: add list_head nf_gc to struct nfsd_file

Commit Message

Chuck Lever Jan. 15, 2025, 7:48 p.m. UTC

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 <[e-mail]>
To: [e-mail], [e-mail], 
[e-mail], [e-mail]

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 <[e-mail]>
---
  fs/nfsd/filecache.c | 18 ++++++++++--------
  fs/nfsd/filecache.h |  1 +
  2 files changed, 11 insertions(+), 8 deletions(-)

Patch

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;
  };