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

Thomas Zimmermann
2025-02-04T08:22:01Z

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.

>
> - 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?

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)