[RFT PATCH v2] tee: shm: Remove refcounting of kernel pages

[Sumit Garg via OP-TEE <op-tee@lists.trustedfirmware.org>]

From: Matthew Wilcox <willy@infradead.org>

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 <m.felsch@pengutronix.de>
Reported-by: Sven Püschel <s.pueschel@pengutronix.de>
Signed-off-by: Matthew Wilcox <willy@infradead.org>
Co-developed-by: Sumit Garg <sumit.garg@oss.qualcomm.com>
Signed-off-by: Sumit Garg <sumit.garg@oss.qualcomm.com>
---

Changes in v2:
- Attribute Matthew as the author of this patch.
- Fix check for user pages.

 drivers/tee/tee_shm.c | 27 ---------------------------
 1 file changed, 27 deletions(-)

diff --git a/drivers/tee/tee_shm.c b/drivers/tee/tee_shm.c
index 4a47de4bb2e5..898707ca21a8 100644
--- a/drivers/tee/tee_shm.c
+++ b/drivers/tee/tee_shm.c
@@ -23,29 +23,11 @@ struct tee_shm_dma_mem {
 	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);
 	}
@@ -477,13 +459,6 @@ register_shm_helper(struct tee_context *ctx, struct iov_iter *iter, u32 flags,
 		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;
@@ -499,8 +474,6 @@ register_shm_helper(struct tee_context *ctx, struct iov_iter *iter, u32 flags,
 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:
-- 
2.51.0