Re: make alloc_anon_inode more useful

[Matthew Wilcox <willy@infradead.org>]

On Tue, Mar 09, 2021 at 04:53:39PM +0100, Christoph Hellwig wrote:
> this series first renames the existing alloc_anon_inode to
> alloc_anon_inode_sb to clearly mark it as requiring a superblock.
> 
> It then adds a new alloc_anon_inode that works on the anon_inode
> file system super block, thus removing tons of boilerplate code.
> 
> The few remainig callers of alloc_anon_inode_sb all use alloc_file_pseudo
> later, but might also be ripe for some cleanup.

On a somewhat related note, could I get you to look at
drivers/video/fbdev/core/fb_defio.c?

As far as I can tell, there's no need for fb_deferred_io_aops to exist.
We could just set file->f_mapping->a_ops to NULL, and set_page_dirty()
would do the exact same thing this code does (except it would get the
return value correct).

But maybe that would make something else go wrong that distinguishes
between page->mapping being NULL and page->mapping->a_ops->foo being NULL?
Completely untested patch ...

diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.c
index a591d291b231..441ec31d3e4d 100644
--- a/drivers/video/fbdev/core/fb_defio.c
+++ b/drivers/video/fbdev/core/fb_defio.c
@@ -151,17 +151,6 @@ static const struct vm_operations_struct fb_deferred_io_vm_ops = {
 	.page_mkwrite	= fb_deferred_io_mkwrite,
 };
 
-static int fb_deferred_io_set_page_dirty(struct page *page)
-{
-	if (!PageDirty(page))
-		SetPageDirty(page);
-	return 0;
-}
-
-static const struct address_space_operations fb_deferred_io_aops = {
-	.set_page_dirty = fb_deferred_io_set_page_dirty,
-};
-
 int fb_deferred_io_mmap(struct fb_info *info, struct vm_area_struct *vma)
 {
 	vma->vm_ops = &fb_deferred_io_vm_ops;
@@ -212,14 +201,6 @@ void fb_deferred_io_init(struct fb_info *info)
 }
 EXPORT_SYMBOL_GPL(fb_deferred_io_init);
 
-void fb_deferred_io_open(struct fb_info *info,
-			 struct inode *inode,
-			 struct file *file)
-{
-	file->f_mapping->a_ops = &fb_deferred_io_aops;
-}
-EXPORT_SYMBOL_GPL(fb_deferred_io_open);
-
 void fb_deferred_io_cleanup(struct fb_info *info)
 {
 	struct fb_deferred_io *fbdefio = info->fbdefio;
diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 06f5805de2de..c4ba76359f22 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -1415,10 +1415,7 @@ __releases(&info->lock)
 		if (res)
 			module_put(info->fbops->owner);
 	}
-#ifdef CONFIG_FB_DEFERRED_IO
-	if (info->fbdefio)
-		fb_deferred_io_open(info, inode, file);
-#endif
+	file->f_mapping->a_ops = NULL;
 out:
 	unlock_fb_info(info);
 	if (res)
diff --git a/include/linux/fb.h b/include/linux/fb.h
index ecfbcc0553a5..a8dccd23c249 100644
--- a/include/linux/fb.h
+++ b/include/linux/fb.h
@@ -659,9 +659,6 @@ static inline void __fb_pad_aligned_buffer(u8 *dst, u32 d_pitch,
 /* drivers/video/fb_defio.c */
 int fb_deferred_io_mmap(struct fb_info *info, struct vm_area_struct *vma);
 extern void fb_deferred_io_init(struct fb_info *info);
-extern void fb_deferred_io_open(struct fb_info *info,
-				struct inode *inode,
-				struct file *file);
 extern void fb_deferred_io_cleanup(struct fb_info *info);
 extern int fb_deferred_io_fsync(struct file *file, loff_t start,
 				loff_t end, int datasync);