Re: [PATCH 1/1] NFS: Fix potential oops in nfs_inode_remove_request()

[Scott Mayhew <smayhew@redhat.com>]

On Tue, 25 Jul 2023, Trond Myklebust wrote:

> On Tue, 2023-07-25 at 11:08 -0400, Scott Mayhew wrote:
> > Once a folio's private data has been cleared, it's possible for
> > another
> > process to clear the folio->mapping (e.g. via
> > invalidate_complete_folio2
> > or evict_mapping_folio), so it wouldn't be safe to call
> > nfs_page_to_inode() after that.
> > 
> > Fixes: 0c493b5cf16e ("NFS: Convert buffered writes to use folios")
> > Signed-off-by: Scott Mayhew <smayhew@redhat.com>
> > ---
> >  fs/nfs/write.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/nfs/write.c b/fs/nfs/write.c
> > index f4cca8f00c0c..489c3f9dae23 100644
> > --- a/fs/nfs/write.c
> > +++ b/fs/nfs/write.c
> > @@ -785,6 +785,8 @@ static void nfs_inode_add_request(struct nfs_page
> > *req)
> >   */
> >  static void nfs_inode_remove_request(struct nfs_page *req)
> >  {
> > +       struct nfs_inode *nfsi = NFS_I(nfs_page_to_inode(req));
> > +
> >         if (nfs_page_group_sync_on_bit(req, PG_REMOVE)) {
> >                 struct folio *folio = nfs_page_to_folio(req-
> > >wb_head);
> >                 struct address_space *mapping =
> > folio_file_mapping(folio);
> > @@ -800,7 +802,7 @@ static void nfs_inode_remove_request(struct
> > nfs_page *req)
> >  
> >         if (test_and_clear_bit(PG_INODE_REF, &req->wb_flags)) {
> >                 nfs_release_request(req);
> > -               atomic_long_dec(&NFS_I(nfs_page_to_inode(req))-
> > >nrequests);
> > +               atomic_long_dec(&nfsi->nrequests);
> 
> Why not just invert the order of the atomic_long_dec() and the
> nfs_release_request()? That way you are also ensuring that the inode is
> still pinned in memory by the open context.

I'm not following.  How does inverting the order prevent the
folio->mapping from getting clobbered?

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