Re: [PATCH] NFSv4/pnfs: Reset the layout state after a layoutreturn

[Trond Myklebust <trondmy@hammerspace.com>]

On Mon, 2025-05-12 at 13:59 -0500, Jeff Layton wrote:
> On Sun, 2025-05-11 at 13:48 +0000, Trond Myklebust wrote:
> > Hi Jeff and Omar,
> > 
> > On Sun, 2025-05-11 at 09:28 -0400, trondmy@kernel.org wrote:
> > > From: Trond Myklebust <trond.myklebust@hammerspace.com>
> > > 
> > > If there are still layout segments in the layout plh_return_lsegs
> > > list
> > > after a layout return, we should be resetting the state to ensure
> > > they
> > > eventually get returned as well.
> > > 
> > > Fixes: 68f744797edd ("pNFS: Do not free layout segments that are
> > > marked for return")
> > > Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
> > > ---
> > >  fs/nfs/pnfs.c | 9 +++++++++
> > >  1 file changed, 9 insertions(+)
> > > 
> > > diff --git a/fs/nfs/pnfs.c b/fs/nfs/pnfs.c
> > > index 10fdd065a61c..fc7c5fb10198 100644
> > > --- a/fs/nfs/pnfs.c
> > > +++ b/fs/nfs/pnfs.c
> > > @@ -745,6 +745,14 @@ pnfs_mark_matching_lsegs_invalid(struct
> > > pnfs_layout_hdr *lo,
> > >   return remaining;
> > >  }
> > >  
> > > +static void pnfs_reset_return_info(struct pnfs_layout_hdr *lo)
> > > +{
> > > + struct pnfs_layout_segment *lseg;
> > > +
> > > + list_for_each_entry(lseg, &lo->plh_return_segs, pls_list)
> > > + pnfs_set_plh_return_info(lo, lseg->pls_range.iomode, 0);
> > > +}
> > > +
> > >  static void
> > >  pnfs_free_returned_lsegs(struct pnfs_layout_hdr *lo,
> > >   struct list_head *free_me,
> > > @@ -1292,6 +1300,7 @@ void pnfs_layoutreturn_free_lsegs(struct
> > > pnfs_layout_hdr *lo,
> > >   pnfs_mark_matching_lsegs_invalid(lo, &freeme, range, seq);
> > >   pnfs_free_returned_lsegs(lo, &freeme, range, seq);
> > >   pnfs_set_layout_stateid(lo, stateid, NULL, true);
> > > + pnfs_reset_return_info(lo);
> > >   } else
> > >   pnfs_mark_layout_stateid_invalid(lo, &freeme);
> > >  out_unlock:
> > 
> > Could the above bug perhaps explain the issue with leaked layout
> > segments that you were seeing?
> > If the client doesn't set NFS_LAYOUT_RETURN_REQUESTED, and the
> > server
> > is unable to recall the layout due to the network getting shut
> > down,
> > then it seems to me that these layout segments just disappear down
> > a
> > black hole.
> > 
> > IOW: the scenario is something like this:
> >  * The client holds a read and a read/write layout.
> >  * The server recalls the read layout.
> >  * The client closes the file while the recall is being processed,
> > so
> >    that the read and read/write layout segments are both put on the
> >    plh_return_segs list.
> >  * The client returns the read layout, and clears the associated
> > read
> >    layout segments. The read/write layout segments are still on the
> >    list, but without NFS_LAYOUT_RETURN_REQUESTED being set.
> > 
> 
> Maybe?
> 
> The problem I think we hit was that pnfs_put_layout_hdr() got called
> and its refcount went to zero while there were still entries on
> plh_return_segs.
> 
> pnfs_put_layout_hdr() calls pnfs_layoutreturn_before_put_layout_hdr()
> as its "last ditch" effort to clean out the plh_return_segs list. It
> looks like your patch will ensure NFS_LAYOUT_RETURN_REQUESTED is set
> on
> all of those entries, but if that flag gets set during the
> pnfs_layoutreturn_before_put_layout_hdr() call, then I think it may
> be
> too late and they'll just leak anyway.
> 
> So I guess the question is: is every entry on plh_return_segs
> guaranteed to get a first attempt at a LAYOUTRETURN before
> pnfs_layoutreturn_before_put_layout_hdr() is called?
> 
> If so, then yes that should fix it. If not, then I think it may not
> (unless I'm misunderstanding this code).

So, the point is that pnfs_layoutreturn_before_put_layout_hdr() will
take a reference to the layout (in pnfs_prepare_layoutreturn()) before
it kicks off the layoutreturn RPC call. Upon finishing the
layoutreturn, the nfs4_layoutreturn_release() callback will then
trigger another layoutreturn attempt when it calls
pnfs_put_layout_hdr(), assuming that NFS_LAYOUT_RETURN_REQUESTED was
set by pnfs_layoutreturn_free_lsegs()->pnfs_reset_return_info().

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