linux-kernel - Re: [PATCH 2/2] i915: do not leak module ref counter

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Date:   Fri, 02 Aug 2019 14:41:15 +0100
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH 2/2] i915: do not leak module ref counter

Quoting Sergey Senozhatsky (2019-08-02 14:35:03)
> On (08/02/19 22:15), Sergey Senozhatsky wrote:
> [..]
> > > > Looking around, it looks like we always need to drop type after
> > > > mounting. Should the
> > > >         put_filesystem(type);
> > > > be here instead?
> > > > 
> > > > Anyway, nice catch.
> > > 
> > > Sigh. put_filesystem() is part of fs internals. I'd be tempted to add
> > 
> > Good catch!
> > 
> > So we can switch to vfs_kern_mount(), I guess, but pass different options,
> > depending on has_transparent_hugepage().
> 
> Hmm. This doesn't look exactly right. It appears that vfs_kern_mount()
> has a slightly different purpose. It's for drivers which register their
> own fstype and fs_context/sb callbacks. A typical usage would be
> 
>         static struct file_system_type nfsd_fs_type = {
>                 .owner→ →       = THIS_MODULE,
>                 .name→  →       = "nfsd",
>                 .init_fs_context = nfsd_init_fs_context,
>                 .kill_sb→       = nfsd_umount,
>         };
>         MODULE_ALIAS_FS("nfsd");
> 
>         vfs_kern_mount(&nfsd_fs_type, SB_KERNMOUNT, "nfsd", NULL);
> 
> i915 is a different beast, it just wants to mount fs and reconfigure
> it, it doesn't want to be an fs. So it seems that current kern_mount()
> is actually right.

struct vfsmount *kern_mount(struct file_system_type *type)
{
        struct vfsmount *mnt;
        mnt = vfs_kern_mount(type, SB_KERNMOUNT, type->name, NULL);
        if (!IS_ERR(mnt)) {
                /*
                 * it is a longterm mount, don't release mnt until
                 * we unmount before file sys is unregistered
                */
                real_mount(mnt)->mnt_ns = MNT_NS_INTERNAL;
        }
        return mnt;
}

With the exception of fiddling with MNT_NS_INTERNAL, it seems
amenable for our needs.
-Chris