Re: [PATCH] vfs: avoid spurious dentry ref/unref cycle on open

[Mateusz Guzik]

On Wed, Aug 7, 2024 at 2:43 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, Aug 07, 2024 at 11:50:50AM +0200, Mateusz Guzik wrote:
>
> > tripping ip:
> > vfs_tmpfile+0x162/0x230:
> > fsnotify_parent at include/linux/fsnotify.h:81
> > (inlined by) fsnotify_file at include/linux/fsnotify.h:131
> > (inlined by) fsnotify_open at include/linux/fsnotify.h:401
> > (inlined by) vfs_tmpfile at fs/namei.c:3781
>
> Try this for incremental; missed the fact that finish_open() is
> used by ->tmpfile() instances, not just ->atomic_open().
>
> Al, crawling back to sleep...
>
> diff --git a/fs/namei.c b/fs/namei.c
> index 95345a5beb3a..0536907e8e79 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -3776,7 +3776,10 @@ int vfs_tmpfile(struct mnt_idmap *idmap,
>         file->f_path.dentry = child;
>         mode = vfs_prepare_mode(idmap, dir, mode, mode, mode);
>         error = dir->i_op->tmpfile(idmap, dir, file, mode);
> -       dput(child);
> +       if (file->f_mode & FMODE_OPENED)
> +               mntget(parentpath->mnt);
> +       else
> +               dput(child);
>         if (file->f_mode & FMODE_OPENED)
>                 fsnotify_open(file);
>         if (error)

That seems to have worked, but my test rig went down due to surprise
PDU maintenance (and it is still down) and I was unable to give the
patch a more serious beating.

-- 
Mateusz Guzik <mjguzik gmail.com>