Re: [PATCH] ceph: Fix a memory leak in ci->i_head_snapc

["Yan, Zheng" <ukernel@gmail.com>]

On Mon, Mar 18, 2019 at 6:33 PM Luis Henriques <lhenriques@suse.com> wrote:
>
> "Yan, Zheng" <ukernel@gmail.com> writes:
>
> > On Fri, Mar 15, 2019 at 7:13 PM Luis Henriques <lhenriques@suse.com> wrote:
> >>
> >> I'm occasionally seeing a kmemleak warning in xfstest generic/013:
> >>
> >> unreferenced object 0xffff8881fccca940 (size 32):
> >>   comm "kworker/0:1", pid 12, jiffies 4295005883 (age 130.648s)
> >>   hex dump (first 32 bytes):
> >>     01 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00  ................
> >>     00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> >>   backtrace:
> >>     [<00000000d741a1ea>] build_snap_context+0x5b/0x2a0
> >>     [<0000000021a00533>] rebuild_snap_realms+0x27/0x90
> >>     [<00000000ac538600>] rebuild_snap_realms+0x42/0x90
> >>     [<000000000e955fac>] ceph_update_snap_trace+0x2ee/0x610
> >>     [<00000000a9550416>] ceph_handle_snap+0x317/0x5f3
> >>     [<00000000fc287b83>] dispatch+0x362/0x176c
> >>     [<00000000a312c741>] ceph_con_workfn+0x9ce/0x2cf0
> >>     [<000000004168e3a9>] process_one_work+0x1d4/0x400
> >>     [<000000002188e9e7>] worker_thread+0x2d/0x3c0
> >>     [<00000000b593e4b3>] kthread+0x112/0x130
> >>     [<00000000a8587dca>] ret_from_fork+0x35/0x40
> >>     [<00000000ba1c9c1d>] 0xffffffffffffffff
> >>
> >> It looks like it is possible that we miss a flush_ack from the MDS when,
> >> for example, umounting the filesystem.  In that case, we can simply drop
> >> the reference to the ceph_snap_context obtained in ceph_queue_cap_snap().
> >>
> >> Link: https://tracker.ceph.com/issues/38224
> >> Cc: stable@vger.kernel.org
> >> Signed-off-by: Luis Henriques <lhenriques@suse.com>
> >> ---
> >>  fs/ceph/caps.c | 7 +++++++
> >>  1 file changed, 7 insertions(+)
> >>
> >> diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
> >> index 36a8dc699448..208f4dc6f574 100644
> >> --- a/fs/ceph/caps.c
> >> +++ b/fs/ceph/caps.c
> >> @@ -1054,6 +1054,7 @@ int ceph_is_any_caps(struct inode *inode)
> >>  static void drop_inode_snap_realm(struct ceph_inode_info *ci)
> >>  {
> >>         struct ceph_snap_realm *realm = ci->i_snap_realm;
> >> +
> >>         spin_lock(&realm->inodes_with_caps_lock);
> >>         list_del_init(&ci->i_snap_realm_item);
> >>         ci->i_snap_realm_counter++;
> >> @@ -1063,6 +1064,12 @@ static void drop_inode_snap_realm(struct ceph_inode_info *ci)
> >>         spin_unlock(&realm->inodes_with_caps_lock);
> >>         ceph_put_snap_realm(ceph_sb_to_client(ci->vfs_inode.i_sb)->mdsc,
> >>                             realm);
> >> +       /*
> >> +        * ci->i_head_snapc should be NULL, but we may still be waiting for a
> >> +        * flush_ack from the MDS.  In that case, we still hold a ref for the
> >> +        * ceph_snap_context and we need to drop it.
> >> +        */
> >> +       ceph_put_snap_context(ci->i_head_snapc);
> >>  }
> >>
> >>  /*
> >
> > This does not seem right.  i_head_snapc is cleared when
> > (ci->i_wrbuffer_ref_head == 0 && ci->i_dirty_caps == 0 &&
> > ci->i_flushing_caps == 0) . Nothing do with dropping ci->i_snap_realm.
> > Did you see 'reconnect denied' during the test? If you did, the fix
> > should be in iterate_session_caps()
> >
>
> No, I didn't saw any 'reconnect denied' in the test.  The test actually
> seems to execute fine, except from the memory leak.
>
> It's very difficult to reproduce this issue, but last time I managed to
> get this memory leak to trigger I actually had some debugging code in
> drop_inode_snap_realm, something like:
>
>   if (ci->i_head_snapc)
>         printk("i_head_snapc: 0x%px\n", ci->i_head_snapc);

please add code that prints i_wrbuffer_ref_head, i_dirty_caps,
i_flushing_caps. and try reproducing it again.


>
> This printk was only executed when the bug triggered (during a
> filesystem umount) and the address shown was the same as in the kmemleak
> warning.
>
> After spending some time looking, I assumed this to be a missing call to
> handle_cap_flush_ack, which would do the i_head_snapc cleanup.
>
> Cheers,
> --
> Luis