Re: [Qemu-devel] [PATCH for-2.9?] 9pfs: fix migration_block leak

[Li Qiang]

Hello,

2017-03-30 23:46 GMT+08:00 Greg Kurz <address@hidden>:

> On Thu, 30 Mar 2017 08:25:25 -0500
> Eric Blake <address@hidden> wrote:
>
> > On 03/30/2017 07:27 AM, Li Qiang wrote:
> > > The guest can leave the pdu->s->migration_blocker exists by attach
> >
> > s/exists/in place/
> > s/attach/attaching/
> >
>

Eric,
Thanks for pointing my mistakes!


> > > but not remove a fid. Then if we hot unplug the 9pfs device, the
> >
>
> In theory you're right, but the current 9p client in linux won't let you
> hot
> unplug the device unless you unmount the 9p share first, hence freeing the
> blocker.
>
>
I think we should consider every possible situation.


> > s/remove/removing/
> >
> > > v9fs_reset() just free the fids, but not free the migration_blocker.
> > > This will leak a memory leak. This patch avoid this.
>
> I had a similar issue sitting my TODO list for quite a time: the blocker
> survives a system_reset. It doesn't cause a memory leak but it prevents
> migration until the guest mounts/unmounts the 9p share again.
>
> This boils down to virtfs_reset() calling free_fid() instead of put_fid()
> IIRC.
>
> >
> > s/leak a/cause a/
> > s/avoid/avoids/
> >
> > >
> > > Signed-off-by: Li Qiang <address@hidden>
> > > ---
> > >  hw/9pfs/9p.c | 6 ++++++
> > >  1 file changed, 6 insertions(+)
> >
> > Probably worth including in 2.9 as a bug fix.
> >
> > Reviewed-by: Eric Blake <address@hidden>
> >
> > >
> > > diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> > > index 48babce..b55c02d 100644
> > > --- a/hw/9pfs/9p.c
> > > +++ b/hw/9pfs/9p.c
> > > @@ -548,6 +548,12 @@ static void coroutine_fn virtfs_reset(V9fsPDU
> *pdu)
> > >              free_fid(pdu, fidp);
> > >          }
> > >      }
> > > +
> > > +    if (pdu->s->migration_blocker) {
> > > +        migrate_del_blocker(pdu->s->migration_blocker);
> > > +        error_free(pdu->s->migration_blocker);
> > > +        pdu->s->migration_blocker = NULL;
> > > +    }
>
> I'd prefer to drain all PDUs in virtfs_reset() and have the loop above
> to call put_fid() instead of free_fid(). If this isn't doable for 2.9,
> I'll apply this patch with a comment.
>
>
Yes, I have considered to use put_fid() to fix this. But I'm not sure the
'fidp->ref' is  at most 1 in virtfs_reset() function(I think it is).

IIUC I think omit the 'else' branch, and call put_fid() directly like this.

diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
index 48babce..ae97e79 100644
--- a/hw/9pfs/9p.c
+++ b/hw/9pfs/9p.c
@@ -544,9 +544,8 @@ static void coroutine_fn virtfs_reset(V9fsPDU *pdu)

         if (fidp->ref) {
             fidp->clunked = 1;
-        } else {
-            free_fid(pdu, fidp);
         }
+        put_fid(pdu, fidp);
     }
 }


If you agree, I will send a formal patch.



> > >  }
> > >
> > >  #define P9_QID_TYPE_DIR         0x80
> > >
> >
>
>