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

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

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.

>
> 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)
>