linux-kernel - Re: [PATCH v2] fbdev: Fix incorrect page mapping clearance at fb_deferred_io

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Date:   Fri, 10 Mar 2023 15:23:08 +0100
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Takashi Iwai <tiwai@...e.de>
Cc:     linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        Patrik Jakobsson <pjakobsson@...e.de>,
        Helge Deller <deller@....de>,
        Miko Larsson <mikoxyzzz@...il.com>
Subject: Re: [PATCH v2] fbdev: Fix incorrect page mapping clearance at
 fb_deferred_io_release()

Added to drm-misc-fixes. Thanks a lot for the patch.

Am 08.03.23 um 11:50 schrieb Takashi Iwai:
> 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.
> 
> As both fb_deferred_io_open() and _close() are called always in the
> fb_info lock (mutex), it's safe to use the normal int for the
> refcounting.
> 
> Also, a useless BUG_ON() is dropped.
> 
> Fixes: 3efc61d95259 ("fbdev: Fix invalid page access after closing deferred I/O devices")
> Cc: <stable@...r.kernel.org>
> Signed-off-by: Takashi Iwai <tiwai@...e.de>
> ---
> v1->v2:
> * Rename to fb_deferred_io_lastclose()
> * Rename the new field from opens to open_count
> * Removed unused variable
> * More comments about fb_info locking
> 
>   drivers/video/fbdev/core/fb_defio.c | 17 +++++++++++++----
>   include/linux/fb.h                  |  1 +
>   2 files changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.c
> index aa5f059d0222..274f5d0fa247 100644
> --- a/drivers/video/fbdev/core/fb_defio.c
> +++ b/drivers/video/fbdev/core/fb_defio.c
> @@ -305,17 +305,18 @@ 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->open_count++;
>   }
>   EXPORT_SYMBOL_GPL(fb_deferred_io_open);
>   
> -void fb_deferred_io_release(struct fb_info *info)
> +static void fb_deferred_io_lastclose(struct fb_info *info)
>   {
> -	struct fb_deferred_io *fbdefio = info->fbdefio;
>   	struct page *page;
>   	int i;
>   
> -	BUG_ON(!fbdefio);
>   	cancel_delayed_work_sync(&info->deferred_work);
>   
>   	/* clear out the mapping that we setup */
> @@ -324,13 +325,21 @@ 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->open_count)
> +		fb_deferred_io_lastclose(info);
> +}
>   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_lastclose(info);
>   
>   	kvfree(info->pagerefs);
>   	mutex_destroy(&fbdefio->lock);
> diff --git a/include/linux/fb.h b/include/linux/fb.h
> index d8d20514ea05..02d09cb57f6c 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 open_count; /* number of opened files; protected by fb_info lock */
>   	struct mutex lock; /* mutex that protects the pageref list */
>   	struct list_head pagereflist; /* list of pagerefs for touched pages */
>   	/* callback */

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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