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

Lorenzo Stoakes
2025-02-04T08:38:08Z

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)
>