linux-kernel - Re: [GIT PULL] Ceph fixes for 5.1-rc7

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Date:   Sun, 28 Apr 2019 17:40:30 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Ilya Dryomov <idryomov@...il.com>, ceph-devel@...r.kernel.org,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        linux-cifs <linux-cifs@...r.kernel.org>
Subject: Re: [GIT PULL] Ceph fixes for 5.1-rc7

On Sun, Apr 28, 2019 at 04:52:16PM +0100, Al Viro wrote:
> On Sun, Apr 28, 2019 at 11:47:58AM -0400, Jeff Layton wrote:
> 
> > We could stick that in ceph_dentry_info (->d_fsdata). We have a flags
> > field in there already.
> 
> Yes, but...  You have it freed in ->d_release(), AFAICS, and without
> any delays.  So lockless accesses will be trouble.

You could RCU-delay the actual kmem_cache_free(ceph_dentry_cachep, di)
in there, but I've no idea whether the overhead would be painful -
on massive eviction (e.g. on memory pressure) it might be.  Another
variant is to introduce ->d_free(), to be called from __d_free()
and __d_free_external().  That, however, would need another ->d_flags
bit for presence of that method, so that we don't get extra overhead
from looking into ->d_op...

Looking through ->d_release() instances, we have

afs: empty, might as well have not been there

autofs: does some sync stuff (eviction from ->active_list/->expire_list)
plus kfree_rcu

ceph: some sync stuff + immediate kmem_cache_free()

debugfs: kfree(), might or might not be worth RCU-delaying

ecryptfs: sync stuff (path_put for ->lower) + RCU-delayed part

fuse: kfree_rcu()

nfs: kfree()

overlayfs: a bunch of dput() (obviously sync) + kfree_rcu()

9p: sync

So it actually might make sense to move the RCU-delayed bits to
separate method.  Some ->d_release() instances would be simply
gone, as for the rest...  I wonder which of the sync parts can
be moved over to ->d_prune().  Not guaranteed to be doable
(or a good idea), but...  E.g. for autofs it almost certainly
would be the right place for the sync parts - we are,
essentially, telling the filesystem to forget its private
(non-refcounted) references to the victim.