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

[Trond Myklebust <trondmy@hammerspace.com>]

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.
>         }
>  }
>  

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