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

On Tue, Feb 04, 2025 at 09:21:55AM +0100, Thomas Zimmermann wrote:

Hi


Am 31.01.25 um 19:28 schrieb Lorenzo Stoakes:

quoted

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 <redacted>
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.

quoted

-	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

quoted

  	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)