Re: [PATCH] fbdev: Fix incorrect page mapping clearance at fb_deferred_io_release()

[Patrik Jakobsson]

On Wed, Mar 8, 2023 at 10:14 AM Takashi Iwai <tiwai@xxxxxxx> wrote:
>
> On Wed, 08 Mar 2023 10:08:24 +0100,
> Patrik Jakobsson wrote:
> >
> > On Wed, Mar 8, 2023 at 7:36 AM Takashi Iwai <tiwai@xxxxxxx> wrote:
> > >
> > > The recent fix for the deferred I/O by the commit
> > >   3efc61d95259 ("fbdev: Fix invalid page access after closing deferred I/O devices")
> > > caused a regression when the same fb device is opened/closed while
> > > it's being used.  It resulted in a frozen screen even if something
> > > is redrawn there after the close.  The breakage is because the patch
> > > was made under a wrong assumption of a single open; in the current
> > > code, fb_deferred_io_release() cleans up the page mapping of the
> > > pageref list and it calls cancel_delayed_work_sync() unconditionally,
> > > where both are no correct behavior for multiple opens.
> > >
> > > This patch adds a refcount for the opens of the device, and applies
> > > the cleanup only when all files get closed.
> > >
> > > Fixes: 3efc61d95259 ("fbdev: Fix invalid page access after closing deferred I/O devices")
> > > Cc: <stable@xxxxxxxxxxxxxxx>
> > > Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
> > > ---
> > >  drivers/video/fbdev/core/fb_defio.c | 16 +++++++++++++---
> > >  include/linux/fb.h                  |  1 +
> > >  2 files changed, 14 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.c
> > > index aa5f059d0222..9dcec9e020b6 100644
> > > --- a/drivers/video/fbdev/core/fb_defio.c
> > > +++ b/drivers/video/fbdev/core/fb_defio.c
> > > @@ -305,17 +305,19 @@ void fb_deferred_io_open(struct fb_info *info,
> > >                          struct inode *inode,
> > >                          struct file *file)
> > >  {
> > > +       struct fb_deferred_io *fbdefio = info->fbdefio;
> > > +
> > >         file->f_mapping->a_ops = &fb_deferred_io_aops;
> > > +       fbdefio->opens++;
> > >  }
> > >  EXPORT_SYMBOL_GPL(fb_deferred_io_open);
> > >
> > > -void fb_deferred_io_release(struct fb_info *info)
> > > +static void fb_deferred_io_release_internal(struct fb_info *info)
> >
> > Maybe a better name would be fb_deferred_io_lastclose() to be more in
> > line with DRM?
>
> Sounds good.
>
> > >  {
> > >         struct fb_deferred_io *fbdefio = info->fbdefio;
> > >         struct page *page;
> > >         int i;
> > >
> > > -       BUG_ON(!fbdefio);
> >
> > Should the BUG_ON be put back into fb_deferred_io_release()?
>
> It can be, but honestly speaking, such a BUG_ON() is utterly useless.
> It should be WARN_ON() and return, if the sanity check is inevitably
> needed.

I agree. It's rather pointless since it's already checked in fb_release().

>
> > >         cancel_delayed_work_sync(&info->deferred_work);
> > >
> > >         /* clear out the mapping that we setup */
> > > @@ -324,13 +326,21 @@ void fb_deferred_io_release(struct fb_info *info)
> > >                 page->mapping = NULL;
> > >         }
> > >  }
> > > +
> > > +void fb_deferred_io_release(struct fb_info *info)
> > > +{
> > > +       struct fb_deferred_io *fbdefio = info->fbdefio;
> > > +
> > > +       if (!--fbdefio->opens)
> > > +               fb_deferred_io_release_internal(info);
> >
> > I think this can race so we need locking.
>
> This one is fine, as it's always called inside the fb lock in the
> caller side.  Maybe worth to comment in the code.

Ah, yes, fb_release() locks around everything. Then we are fine. A
comment would be nice.

>
> > > +}
> > >  EXPORT_SYMBOL_GPL(fb_deferred_io_release);
> > >
> > >  void fb_deferred_io_cleanup(struct fb_info *info)
> > >  {
> > >         struct fb_deferred_io *fbdefio = info->fbdefio;
> > >
> > > -       fb_deferred_io_release(info);
> > > +       fb_deferred_io_release_internal(info);
> > >
> > >         kvfree(info->pagerefs);
> > >         mutex_destroy(&fbdefio->lock);
> > > diff --git a/include/linux/fb.h b/include/linux/fb.h
> > > index d8d20514ea05..29674a29d1c4 100644
> > > --- a/include/linux/fb.h
> > > +++ b/include/linux/fb.h
> > > @@ -212,6 +212,7 @@ struct fb_deferred_io {
> > >         /* delay between mkwrite and deferred handler */
> > >         unsigned long delay;
> > >         bool sort_pagereflist; /* sort pagelist by offset */
> > > +       int opens; /* number of opened files */
> >
> > I would prefer the name num_opens (or open_count as in DRM) instead of
> > opens since it can be interpreted as a verb.
>
> I don't mind either way.  I'd choose the latter.
>
> > Also, don't we need it to be atomic_t?
>
> It's always in the fb lock, so that should be fine with the standard
> int.

Yes

>
>
> thanks,
>
> Takashi