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

Lorenzo Stoakes
2025-01-31T18:30:33Z

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");

- 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;
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);
--
2.48.1