Re: [PATCH RFC] nfsd: avoid recursive locking through fsnotify

[Trond Myklebust <trondmy@hammerspace.com>]

On Fri, 2022-03-18 at 17:16 -0700, Khazhismel Kumykov wrote:
> 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.

Isn't that stack trace showing a slab direct reclaim, and not a
filesystem writeback situation?

Does memalloc_nofs_save()/restore() really fix this problem? It seems
to me that it cannot, particularly since knfsd is not a filesystem, and
so does not ever handle writeback of dirty pages.

Cc: the linux-mm mailing list in search of answers to the above 2
questions.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com