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

[Patrik Jakobsson]

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?

>  {
>         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()?

>         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.

> +}
>  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.
Also, don't we need it to be atomic_t?

>         struct mutex lock; /* mutex that protects the pageref list */
>         struct list_head pagereflist; /* list of pagerefs for touched pages */
>         /* callback */
> --
> 2.35.3
>