Re: [PATCH 3/3] fb_defio: do not use deprecated page->mapping, index fields

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi


Am 04.02.25 um 09:37 schrieb Lorenzo Stoakes:
> On Tue, Feb 04, 2025 at 09:21:55AM +0100, Thomas Zimmermann wrote:
>> Hi
>>
>>
>> Am 31.01.25 um 19:28 schrieb Lorenzo Stoakes:
>>> With the introduction of mapping_wrprotect_page() there is no need to use
>>> folio_mkclean() in order to write-protect mappings of frame buffer pages,
>>> and therefore no need to inappropriately set kernel-allocated page->index,
>>> mapping fields to permit this operation.
>>>
>>> Instead, store the pointer to the page cache object for the mapped driver
>>> in the fb_deferred_io object, and use the already stored page offset from
>>> the pageref object to look up mappings in order to write-protect them.
>>>
>>> This is justified, as for the page objects to store a mapping pointer at
>>> the point of assignment of pages, they must all reference the same
>>> underlying address_space object. Since the life time of the pagerefs is
>>> also the lifetime of the fb_deferred_io object, storing the pointer here
>>> makes snese.
>>>
>>> This eliminates the need for all of the logic around setting and
>>> maintaining page->index,mapping which we remove.
>>>
>>> This eliminates the use of folio_mkclean() entirely but otherwise should
>>> have no functional change.
>>>
>>> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>>> Tested-by: Kajtar Zsolt <soci@c64.rulez.org>
>>> ---
>>>    drivers/video/fbdev/core/fb_defio.c | 38 +++++++++--------------------
>>>    include/linux/fb.h                  |  1 +
>>>    2 files changed, 12 insertions(+), 27 deletions(-)
>>>
>>> diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.c
>>> index 65363df8e81b..b9bab27a8c0f 100644
>>> --- a/drivers/video/fbdev/core/fb_defio.c
>>> +++ b/drivers/video/fbdev/core/fb_defio.c
>>> @@ -69,14 +69,6 @@ static struct fb_deferred_io_pageref *fb_deferred_io_pageref_lookup(struct fb_in
>>>    	return pageref;
>>>    }
>>> -static void fb_deferred_io_pageref_clear(struct fb_deferred_io_pageref *pageref)
>>> -{
>>> -	struct page *page = pageref->page;
>>> -
>>> -	if (page)
>>> -		page->mapping = NULL;
>>> -}
>>> -
>>>    static struct fb_deferred_io_pageref *fb_deferred_io_pageref_get(struct fb_info *info,
>>>    								 unsigned long offset,
>>>    								 struct page *page)
>>> @@ -140,13 +132,10 @@ static vm_fault_t fb_deferred_io_fault(struct vm_fault *vmf)
>>>    	if (!page)
>>>    		return VM_FAULT_SIGBUS;
>>> -	if (vmf->vma->vm_file)
>>> -		page->mapping = vmf->vma->vm_file->f_mapping;
>>> -	else
>>> +	if (!vmf->vma->vm_file)
>>>    		printk(KERN_ERR "no mapping available\n");
>> fb_err() here.
> Ack, will fix on respin.
>
>>> -	BUG_ON(!page->mapping);
>>> -	page->index = vmf->pgoff; /* for folio_mkclean() */
>>> +	BUG_ON(!info->fbdefio->mapping);
>>>    	vmf->page = page;
>>>    	return 0;
>>> @@ -194,9 +183,9 @@ static vm_fault_t fb_deferred_io_track_page(struct fb_info *info, unsigned long
>>>    	/*
>>>    	 * We want the page to remain locked from ->page_mkwrite until
>>> -	 * the PTE is marked dirty to avoid folio_mkclean() being called
>>> -	 * before the PTE is updated, which would leave the page ignored
>>> -	 * by defio.
>>> +	 * the PTE is marked dirty to avoid mapping_wrprotect_page()
>>> +	 * being called before the PTE is updated, which would leave
>>> +	 * the page ignored by defio.
>>>    	 * Do this by locking the page here and informing the caller
>>>    	 * about it with VM_FAULT_LOCKED.
>>>    	 */
>>> @@ -274,14 +263,13 @@ static void fb_deferred_io_work(struct work_struct *work)
>>>    	struct fb_deferred_io_pageref *pageref, *next;
>>>    	struct fb_deferred_io *fbdefio = info->fbdefio;
>>> -	/* here we mkclean the pages, then do all deferred IO */
>>> +	/* here we wrprotect the page's mappings, then do all deferred IO. */
>>>    	mutex_lock(&fbdefio->lock);
>>>    	list_for_each_entry(pageref, &fbdefio->pagereflist, list) {
>>> -		struct folio *folio = page_folio(pageref->page);
>>> +		struct page *page = pageref->page;
>>> +		pgoff_t pgoff = pageref->offset >> PAGE_SHIFT;
>>> -		folio_lock(folio);
>>> -		folio_mkclean(folio);
>>> -		folio_unlock(folio);
>>> +		mapping_wrprotect_page(fbdefio->mapping, pgoff, 1, page);
>>>    	}
>>>    	/* driver's callback with pagereflist */
>>> @@ -337,6 +325,7 @@ void fb_deferred_io_open(struct fb_info *info,
>>>    {
>>>    	struct fb_deferred_io *fbdefio = info->fbdefio;
>>> +	fbdefio->mapping = file->f_mapping;
>> Does this still work if more than one program opens the file?
> Yes, the mapping (address_space) pointer will remain the same across the
> board. The way defio is implemented absolutely relies on this assumption.

Great. With the fb_err() fixed, you can add

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>

to the patch.

Best regards
Thomas

>
>> Best regard
>> Thomas
>>
>>>    	file->f_mapping->a_ops = &fb_deferred_io_aops;
>>>    	fbdefio->open_count++;
>>>    }
>>> @@ -344,13 +333,7 @@ EXPORT_SYMBOL_GPL(fb_deferred_io_open);
>>>    static void fb_deferred_io_lastclose(struct fb_info *info)
>>>    {
>>> -	unsigned long i;
>>> -
>>>    	flush_delayed_work(&info->deferred_work);
>>> -
>>> -	/* clear out the mapping that we setup */
>>> -	for (i = 0; i < info->npagerefs; ++i)
>>> -		fb_deferred_io_pageref_clear(&info->pagerefs[i]);
>>>    }
>>>    void fb_deferred_io_release(struct fb_info *info)
>>> @@ -370,5 +353,6 @@ void fb_deferred_io_cleanup(struct fb_info *info)
>>>    	kvfree(info->pagerefs);
>>>    	mutex_destroy(&fbdefio->lock);
>>> +	fbdefio->mapping = NULL;
>>>    }
>>>    EXPORT_SYMBOL_GPL(fb_deferred_io_cleanup);
>>> diff --git a/include/linux/fb.h b/include/linux/fb.h
>>> index 5ba187e08cf7..cd653862ab99 100644
>>> --- a/include/linux/fb.h
>>> +++ b/include/linux/fb.h
>>> @@ -225,6 +225,7 @@ struct fb_deferred_io {
>>>    	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 */
>>> +	struct address_space *mapping; /* page cache object for fb device */
>>>    	/* callback */
>>>    	struct page *(*get_page)(struct fb_info *info, unsigned long offset);
>>>    	void (*deferred_io)(struct fb_info *info, struct list_head *pagelist);
>> --
>> --
>> Thomas Zimmermann
>> Graphics Driver Developer
>> SUSE Software Solutions Germany GmbH
>> Frankenstrasse 146, 90461 Nuernberg, Germany
>> GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
>> HRB 36809 (AG Nuernberg)
>>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)