tee: shm: Remove refcounting of kernel pages

Earlier TEE subsystem assumed to refcount all the memory pages to be
shared with TEE implementation to be refcounted. However, the slab
allocations within the kernel don't allow refcounting kernel pages.

It is rather better to trust the kernel clients to not free pages while
being shared with TEE implementation. Hence, remove refcounting of kernel
pages from register_shm_helper() API.

Fixes: b9c0e49abfca ("mm: decline to manipulate the refcount on a slab page")
Reported-by: Marco Felsch <[e-mail]>
Reported-by: Sven Püschel <[e-mail]>
Signed-off-by: Matthew Wilcox <[e-mail]>
Co-developed-by: Sumit Garg <[e-mail]>
Signed-off-by: Sumit Garg <[e-mail]>
Tested-by: Sven Püschel <[e-mail]>
Signed-off-by: Jens Wiklander <[e-mail]>

drivers/tee/tee_shm.c

···
 	struct page *page;
 };
 
-static void shm_put_kernel_pages(struct page **pages, size_t page_count)
-{
-	size_t n;
-
-	for (n = 0; n < page_count; n++)
-		put_page(pages[n]);
-}
-
-static void shm_get_kernel_pages(struct page **pages, size_t page_count)
-{
-	size_t n;
-
-	for (n = 0; n < page_count; n++)
-		get_page(pages[n]);
-}
-
 static void release_registered_pages(struct tee_shm *shm)
 {
 	if (shm->pages) {
 		if (shm->flags & TEE_SHM_USER_MAPPED)
 			unpin_user_pages(shm->pages, shm->num_pages);
-		else
-			shm_put_kernel_pages(shm->pages, shm->num_pages);
 
 		kfree(shm->pages);
 	}
···
 		goto err_put_shm_pages;
 	}
 
-	/*
-	 * iov_iter_extract_kvec_pages does not get reference on the pages,
-	 * get a reference on them.
-	 */
-	if (iov_iter_is_kvec(iter))
-		shm_get_kernel_pages(shm->pages, num_pages);
-
 	shm->offset = off;
 	shm->size = len;
 	shm->num_pages = num_pages;
···
 err_put_shm_pages:
 	if (!iov_iter_is_kvec(iter))
 		unpin_user_pages(shm->pages, shm->num_pages);
-	else
-		shm_put_kernel_pages(shm->pages, shm->num_pages);
 err_free_shm_pages:
 	kfree(shm->pages);
 err_free_shm: