drm/ttm: Remove the struct ttm_backup abstraction

The abstraction was previously added to support separate
ttm_backup implementations.

However with the current implementation casting from a
struct file to a struct ttm_backup, we run into trouble since
struct file may have randomized the layout and gcc complains.

Remove the struct ttm_backup abstraction

Cc: [e-mail]
Cc: Matthew Brost <[e-mail]>
Cc: Dave Airlie <[e-mail]>
Cc: Christian König <[e-mail]>
Cc: Matthew Auld <[e-mail]>
Cc: Al Viro <[e-mail]>
Reported-by: Kees Cook <[e-mail]>
Closes: https://lore.kernel.org/dri-devel/[e-mail]/T/#mb153ab9216cb813b92bdeb36f391ad4808c2ba29
Suggested-by: Christian König <[e-mail]>
Fixes: 70d645deac98 ("drm/ttm: Add helpers for shrinking")
Signed-off-by: Thomas Hellström <[e-mail]>
Reviewed-by: Christian König <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/gpu/drm/ttm/ttm_backup.c

···
 #include <linux/swap.h>
 
 /*
- * Casting from randomized struct file * to struct ttm_backup * is fine since
- * struct ttm_backup is never defined nor dereferenced.
- */
-static struct file *ttm_backup_to_file(struct ttm_backup *backup)
-{
-	return (void *)backup;
-}
-
-static struct ttm_backup *ttm_file_to_backup(struct file *file)
-{
-	return (void *)file;
-}
-
-/*
  * Need to map shmem indices to handle since a handle value
  * of 0 means error, following the swp_entry_t convention.
  */
···
  * @backup: The struct backup pointer used to obtain the handle
  * @handle: The handle obtained from the @backup_page function.
  */
-void ttm_backup_drop(struct ttm_backup *backup, pgoff_t handle)
+void ttm_backup_drop(struct file *backup, pgoff_t handle)
 {
 	loff_t start = ttm_backup_handle_to_shmem_idx(handle);
 
 	start <<= PAGE_SHIFT;
-	shmem_truncate_range(file_inode(ttm_backup_to_file(backup)), start,
+	shmem_truncate_range(file_inode(backup), start,
 			     start + PAGE_SIZE - 1);
 }
 
···
  * Return: 0 on success, Negative error code on failure, notably
  * -EINTR if @intr was set to true and a signal is pending.
  */
-int ttm_backup_copy_page(struct ttm_backup *backup, struct page *dst,
+int ttm_backup_copy_page(struct file *backup, struct page *dst,
 			 pgoff_t handle, bool intr)
 {
-	struct file *filp = ttm_backup_to_file(backup);
-	struct address_space *mapping = filp->f_mapping;
+	struct address_space *mapping = backup->f_mapping;
 	struct folio *from_folio;
 	pgoff_t idx = ttm_backup_handle_to_shmem_idx(handle);
 
···
  * the folio size- and usage.
  */
 s64
-ttm_backup_backup_page(struct ttm_backup *backup, struct page *page,
+ttm_backup_backup_page(struct file *backup, struct page *page,
 		       bool writeback, pgoff_t idx, gfp_t page_gfp,
 		       gfp_t alloc_gfp)
 {
-	struct file *filp = ttm_backup_to_file(backup);
-	struct address_space *mapping = filp->f_mapping;
+	struct address_space *mapping = backup->f_mapping;
 	unsigned long handle = 0;
 	struct folio *to_folio;
 	int ret;
···
  *
  * After a call to this function, it's illegal to use the @backup pointer.
  */
-void ttm_backup_fini(struct ttm_backup *backup)
+void ttm_backup_fini(struct file *backup)
 {
-	fput(ttm_backup_to_file(backup));
+	fput(backup);
 }
 
 /**
···
  *
  * Create a backup utilizing shmem objects.
  *
- * Return: A pointer to a struct ttm_backup on success,
+ * Return: A pointer to a struct file on success,
  * an error pointer on error.
  */
-struct ttm_backup *ttm_backup_shmem_create(loff_t size)
+struct file *ttm_backup_shmem_create(loff_t size)
 {
-	struct file *filp;
-
-	filp = shmem_file_setup("ttm shmem backup", size, 0);
-
-	return ttm_file_to_backup(filp);
+	return shmem_file_setup("ttm shmem backup", size, 0);
 }

drivers/gpu/drm/ttm/ttm_pool.c

···
  * if successful, populate the page-table and dma-address arrays.
  */
 static int ttm_pool_restore_commit(struct ttm_pool_tt_restore *restore,
-				   struct ttm_backup *backup,
+				   struct file *backup,
 				   const struct ttm_operation_ctx *ctx,
 				   struct ttm_pool_alloc_state *alloc)
 
···
 				pgoff_t start_page, pgoff_t end_page)
 {
 	struct page **pages = &tt->pages[start_page];
-	struct ttm_backup *backup = tt->backup;
+	struct file *backup = tt->backup;
 	pgoff_t i, nr;
 
 	for (i = start_page; i < end_page; i += nr, pages += nr) {
···
 long ttm_pool_backup(struct ttm_pool *pool, struct ttm_tt *tt,
 		     const struct ttm_backup_flags *flags)
 {
-	struct ttm_backup *backup = tt->backup;
+	struct file *backup = tt->backup;
 	struct page *page;
 	unsigned long handle;
 	gfp_t alloc_gfp;

drivers/gpu/drm/ttm/ttm_tt.c

···
  */
 int ttm_tt_setup_backup(struct ttm_tt *tt)
 {
-	struct ttm_backup *backup =
+	struct file *backup =
 		ttm_backup_shmem_create(((loff_t)tt->num_pages) << PAGE_SHIFT);
 
 	if (WARN_ON_ONCE(!(tt->page_flags & TTM_TT_FLAG_EXTERNAL_MAPPABLE)))

include/drm/ttm/ttm_backup.h

···
 #include <linux/mm_types.h>
 #include <linux/shmem_fs.h>
 
-struct ttm_backup;
-
 /**
  * ttm_backup_handle_to_page_ptr() - Convert handle to struct page pointer
  * @handle: The handle to convert.
···
 	return (unsigned long)page >> 1;
 }
 
-void ttm_backup_drop(struct ttm_backup *backup, pgoff_t handle);
+void ttm_backup_drop(struct file *backup, pgoff_t handle);
 
-int ttm_backup_copy_page(struct ttm_backup *backup, struct page *dst,
+int ttm_backup_copy_page(struct file *backup, struct page *dst,
 			 pgoff_t handle, bool intr);
 
 s64
-ttm_backup_backup_page(struct ttm_backup *backup, struct page *page,
+ttm_backup_backup_page(struct file *backup, struct page *page,
 		       bool writeback, pgoff_t idx, gfp_t page_gfp,
 		       gfp_t alloc_gfp);
 
-void ttm_backup_fini(struct ttm_backup *backup);
+void ttm_backup_fini(struct file *backup);
 
 u64 ttm_backup_bytes_avail(void);
 
-struct ttm_backup *ttm_backup_shmem_create(loff_t size);
+struct file *ttm_backup_shmem_create(loff_t size);
 
 #endif

include/drm/ttm/ttm_tt.h

···
 	 * ttm_tt_create() callback is responsible for assigning
 	 * this field.
 	 */
-	struct ttm_backup *backup;
+	struct file *backup;
 	/**
 	 * @caching: The current caching state of the pages, see enum
 	 * ttm_caching.