[PATCH RFC] nfsd: avoid recursive locking through fsnotify

[Khazhismel Kumykov <khazhy@google.com>]

fsnotify_add_inode_mark may allocate with GFP_KERNEL, which may result
in recursing back into nfsd, resulting in deadlock. See below stack.

nfsd            D    0 1591536      2 0x80004080
Call Trace:
 __schedule+0x497/0x630
 schedule+0x67/0x90
 schedule_preempt_disabled+0xe/0x10
 __mutex_lock+0x347/0x4b0
 fsnotify_destroy_mark+0x22/0xa0
 nfsd_file_free+0x79/0xd0 [nfsd]
 nfsd_file_put_noref+0x7c/0x90 [nfsd]
 nfsd_file_lru_dispose+0x6d/0xa0 [nfsd]
 nfsd_file_lru_scan+0x57/0x80 [nfsd]
 do_shrink_slab+0x1f2/0x330
 shrink_slab+0x244/0x2f0
 shrink_node+0xd7/0x490
 do_try_to_free_pages+0x12f/0x3b0
 try_to_free_pages+0x43f/0x540
 __alloc_pages_slowpath+0x6ab/0x11c0
 __alloc_pages_nodemask+0x274/0x2c0
 alloc_slab_page+0x32/0x2e0
 new_slab+0xa6/0x8b0
 ___slab_alloc+0x34b/0x520
 kmem_cache_alloc+0x1c4/0x250
 fsnotify_add_mark_locked+0x18d/0x4c0
 fsnotify_add_mark+0x48/0x70
 nfsd_file_acquire+0x570/0x6f0 [nfsd]
 nfsd_read+0xa7/0x1c0 [nfsd]
 nfsd3_proc_read+0xc1/0x110 [nfsd]
 nfsd_dispatch+0xf7/0x240 [nfsd]
 svc_process_common+0x2f4/0x610 [sunrpc]
 svc_process+0xf9/0x110 [sunrpc]
 nfsd+0x10e/0x180 [nfsd]
 kthread+0x130/0x140
 ret_from_fork+0x35/0x40

Signed-off-by: Khazhismel Kumykov <khazhy@google.com>
---
 fs/nfsd/filecache.c | 4 ++++
 1 file changed, 4 insertions(+)

Marking this RFC since I haven't actually had a chance to test this, we
we're seeing this deadlock for some customers.

diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
index fdf89fcf1a0c..a14760f9b486 100644
--- a/fs/nfsd/filecache.c
+++ b/fs/nfsd/filecache.c
@@ -121,6 +121,7 @@ nfsd_file_mark_find_or_create(struct nfsd_file *nf)
 	struct fsnotify_mark	*mark;
 	struct nfsd_file_mark	*nfm = NULL, *new;
 	struct inode *inode = nf->nf_inode;
+	unsigned int pflags;
 
 	do {
 		mutex_lock(&nfsd_file_fsnotify_group->mark_mutex);
@@ -149,7 +150,10 @@ nfsd_file_mark_find_or_create(struct nfsd_file *nf)
 		new->nfm_mark.mask = FS_ATTRIB|FS_DELETE_SELF;
 		refcount_set(&new->nfm_ref, 1);
 
+		/* fsnotify allocates, avoid recursion back into nfsd */
+		pflags = memalloc_nofs_save();
 		err = fsnotify_add_inode_mark(&new->nfm_mark, inode, 0);
+		memalloc_nofs_restore(pflags);
 
 		/*
 		 * If the add was successful, then return the object.
-- 
2.35.1.894.gb6a874cedc-goog