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

[Khazhy Kumykov <khazhy@google.com>]

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On Sat, Mar 19, 2022 at 2:36 AM Amir Goldstein <amir73il@gmail.com> wrote:
>
> On Sat, Mar 19, 2022 at 9:02 AM Trond Myklebust <trondmy@hammerspace.com> wrote:
> >
> > 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.
> >
>
> Maybe NOFS throttles direct reclaims to the point that the problem is
> harder to hit?

(I think I simply got confused - I don't see reason that NOFS would
help with direct reclaim, though it does look like the gfp flags are
passed via a shrink_control struct so one *could* react to them in the
shrinker - again not an area i'm super familiar with)

>
> This report came in at good timing for me.
>
> It demonstrates an issue I did not predict for "volatile"' fanotify marks [1].
> As far as I can tell, nfsd filecache is currently the only fsnotify backend that
> frees fsnotify marks in memory shrinker. "volatile" fanotify marks would also
> be evictable in that way, so they would expose fanotify to this deadlock.
>
> For the short term, maybe nfsd filecache can avoid the problem by checking
> mutex_is_locked(&nfsd_file_fsnotify_group->mark_mutex) and abort the
> shrinker. I wonder if there is a place for a helper mutex_is_locked_by_me()?

fwiw, it does look like ~5.5 nfsd did stop freeing fanotify marks
during reclaim, in the commit "nfsd: Containerise filecache
laundrette" (I had sent an earlier email about this, not sure where
that's getting caught up, but I do see it on lore...)


>
> Jan,
>
> A relatively simple fix would be to allocate fsnotify_mark_connector in
> fsnotify_add_mark() and free it, if a connector already exists for the object.
> I don't think there is a good reason to optimize away this allocation
> for the case of a non-first group to set a mark on an object?
>
> Thanks,
> Amir.
>
>
>
> [1] https://lore.kernel.org/linux-fsdevel/20220307155741.1352405-1-amir73il@gmail.com/

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