fscrypt: simplify bounce page handling

Currently, bounce page handling for writes to encrypted files is
unnecessarily complicated.  A fscrypt_ctx is allocated along with each
bounce page, page_private(bounce_page) points to this fscrypt_ctx, and
fscrypt_ctx::w::control_page points to the original pagecache page.

However, because writes don't use the fscrypt_ctx for anything else,
there's no reason why page_private(bounce_page) can't just point to the
original pagecache page directly.

Therefore, this patch makes this change.  In the process, it also cleans
up the API exposed to filesystems that allows testing whether a page is
a bounce page, getting the pagecache page from a bounce page, and
freeing a bounce page.

Reviewed-by: Chandan Rajendra <chandan@linux.ibm.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>

3 files changed, 40 insertions, 106 deletions

diff --git a/fs/crypto/bio.c b/fs/crypto/bio.c
index b46021ebde85..c857b70b5328 100644
--- a/fs/crypto/bio.c
+++ b/fs/crypto/bio.c
@@ -70,46 +70,18 @@ void fscrypt_enqueue_decrypt_bio(struct fscrypt_ctx *ctx, struct bio *bio)
 }
 EXPORT_SYMBOL(fscrypt_enqueue_decrypt_bio);
 
-void fscrypt_pullback_bio_page(struct page **page, bool restore)
-{
-	struct fscrypt_ctx *ctx;
-	struct page *bounce_page;
-
-	/* The bounce data pages are unmapped. */
-	if ((*page)->mapping)
-		return;
-
-	/* The bounce data page is unmapped. */
-	bounce_page = *page;
-	ctx = (struct fscrypt_ctx *)page_private(bounce_page);
-
-	/* restore control page */
-	*page = ctx->w.control_page;
-
-	if (restore)
-		fscrypt_restore_control_page(bounce_page);
-}
-EXPORT_SYMBOL(fscrypt_pullback_bio_page);
-
 int fscrypt_zeroout_range(const struct inode *inode, pgoff_t lblk,
 				sector_t pblk, unsigned int len)
 {
-	struct fscrypt_ctx *ctx;
-	struct page *ciphertext_page = NULL;
+	struct page *ciphertext_page;
 	struct bio *bio;
 	int ret, err = 0;
 
 	BUG_ON(inode->i_sb->s_blocksize != PAGE_SIZE);
 
-	ctx = fscrypt_get_ctx(GFP_NOFS);
-	if (IS_ERR(ctx))
-		return PTR_ERR(ctx);
-
-	ciphertext_page = fscrypt_alloc_bounce_page(ctx, GFP_NOWAIT);
-	if (IS_ERR(ciphertext_page)) {
-		err = PTR_ERR(ciphertext_page);
-		goto errout;
-	}
+	ciphertext_page = fscrypt_alloc_bounce_page(GFP_NOWAIT);
+	if (!ciphertext_page)
+		return -ENOMEM;
 
 	while (len--) {
 		err = fscrypt_do_page_crypto(inode, FS_ENCRYPT, lblk,
@@ -147,7 +119,7 @@ int fscrypt_zeroout_range(const struct inode *inode, pgoff_t lblk,
 	}
 	err = 0;
 errout:
-	fscrypt_release_ctx(ctx);
+	fscrypt_free_bounce_page(ciphertext_page);
 	return err;
 }
 EXPORT_SYMBOL(fscrypt_zeroout_range);
diff --git a/fs/crypto/crypto.c b/fs/crypto/crypto.c
index 335a362ee446..881e2a69f8a6 100644
--- a/fs/crypto/crypto.c
+++ b/fs/crypto/crypto.c
@@ -64,18 +64,11 @@ EXPORT_SYMBOL(fscrypt_enqueue_decrypt_work);
  *
  * If the encryption context was allocated from the pre-allocated pool, returns
  * it to that pool. Else, frees it.
- *
- * If there's a bounce page in the context, this frees that.
  */
 void fscrypt_release_ctx(struct fscrypt_ctx *ctx)
 {
 	unsigned long flags;
 
-	if (ctx->flags & FS_CTX_HAS_BOUNCE_BUFFER_FL && ctx->w.bounce_page) {
-		mempool_free(ctx->w.bounce_page, fscrypt_bounce_page_pool);
-		ctx->w.bounce_page = NULL;
-	}
-	ctx->w.control_page = NULL;
 	if (ctx->flags & FS_CTX_REQUIRES_FREE_ENCRYPT_FL) {
 		kmem_cache_free(fscrypt_ctx_cachep, ctx);
 	} else {
@@ -100,14 +93,8 @@ struct fscrypt_ctx *fscrypt_get_ctx(gfp_t gfp_flags)
 	unsigned long flags;
 
 	/*
-	 * We first try getting the ctx from a free list because in
-	 * the common case the ctx will have an allocated and
-	 * initialized crypto tfm, so it's probably a worthwhile
-	 * optimization. For the bounce page, we first try getting it
-	 * from the kernel allocator because that's just about as fast
-	 * as getting it from a list and because a cache of free pages
-	 * should generally be a "last resort" option for a filesystem
-	 * to be able to do its job.
+	 * First try getting a ctx from the free list so that we don't have to
+	 * call into the slab allocator.
 	 */
 	spin_lock_irqsave(&fscrypt_ctx_lock, flags);
 	ctx = list_first_entry_or_null(&fscrypt_free_ctxs,
@@ -123,11 +110,31 @@ struct fscrypt_ctx *fscrypt_get_ctx(gfp_t gfp_flags)
 	} else {
 		ctx->flags &= ~FS_CTX_REQUIRES_FREE_ENCRYPT_FL;
 	}
-	ctx->flags &= ~FS_CTX_HAS_BOUNCE_BUFFER_FL;
 	return ctx;
 }
 EXPORT_SYMBOL(fscrypt_get_ctx);
 
+struct page *fscrypt_alloc_bounce_page(gfp_t gfp_flags)
+{
+	return mempool_alloc(fscrypt_bounce_page_pool, gfp_flags);
+}
+
+/**
+ * fscrypt_free_bounce_page() - free a ciphertext bounce page
+ *
+ * Free a bounce page that was allocated by fscrypt_encrypt_page(), or by
+ * fscrypt_alloc_bounce_page() directly.
+ */
+void fscrypt_free_bounce_page(struct page *bounce_page)
+{
+	if (!bounce_page)
+		return;
+	set_page_private(bounce_page, (unsigned long)NULL);
+	ClearPagePrivate(bounce_page);
+	mempool_free(bounce_page, fscrypt_bounce_page_pool);
+}
+EXPORT_SYMBOL(fscrypt_free_bounce_page);
+
 void fscrypt_generate_iv(union fscrypt_iv *iv, u64 lblk_num,
 			 const struct fscrypt_info *ci)
 {
@@ -186,16 +193,6 @@ int fscrypt_do_page_crypto(const struct inode *inode, fscrypt_direction_t rw,
 	return 0;
 }
 
-struct page *fscrypt_alloc_bounce_page(struct fscrypt_ctx *ctx,
-				       gfp_t gfp_flags)
-{
-	ctx->w.bounce_page = mempool_alloc(fscrypt_bounce_page_pool, gfp_flags);
-	if (ctx->w.bounce_page == NULL)
-		return ERR_PTR(-ENOMEM);
-	ctx->flags |= FS_CTX_HAS_BOUNCE_BUFFER_FL;
-	return ctx->w.bounce_page;
-}
-
 /**
  * fscypt_encrypt_page() - Encrypts a page
  * @inode:     The inode for which the encryption should take place
@@ -210,22 +207,12 @@ struct page *fscrypt_alloc_bounce_page(struct fscrypt_ctx *ctx,
  *             previously written data.
  * @gfp_flags: The gfp flag for memory allocation
  *
- * Encrypts @page using the ctx encryption context. Performs encryption
- * either in-place or into a newly allocated bounce page.
- * Called on the page write path.
- *
- * Bounce page allocation is the default.
- * In this case, the contents of @page are encrypted and stored in an
- * allocated bounce page. @page has to be locked and the caller must call
- * fscrypt_restore_control_page() on the returned ciphertext page to
- * release the bounce buffer and the encryption context.
- *
- * In-place encryption is used by setting the FS_CFLG_OWN_PAGES flag in
- * fscrypt_operations. Here, the input-page is returned with its content
- * encrypted.
+ * Encrypts @page.  If the filesystem set FS_CFLG_OWN_PAGES, then the data is
+ * encrypted in-place and @page is returned.  Else, a bounce page is allocated,
+ * the data is encrypted into the bounce page, and the bounce page is returned.
+ * The caller is responsible for calling fscrypt_free_bounce_page().
  *
- * Return: A page with the encrypted content on success. Else, an
- * error value or NULL.
+ * Return: A page containing the encrypted data on success, else an ERR_PTR()
  */
 struct page *fscrypt_encrypt_page(const struct inode *inode,
 				struct page *page,
@@ -234,7 +221,6 @@ struct page *fscrypt_encrypt_page(const struct inode *inode,
 				u64 lblk_num, gfp_t gfp_flags)
 
 {
-	struct fscrypt_ctx *ctx;
 	struct page *ciphertext_page = page;
 	int err;
 
@@ -253,30 +239,20 @@ struct page *fscrypt_encrypt_page(const struct inode *inode,
 
 	BUG_ON(!PageLocked(page));
 
-	ctx = fscrypt_get_ctx(gfp_flags);
-	if (IS_ERR(ctx))
-		return ERR_CAST(ctx);
-
 	/* The encryption operation will require a bounce page. */
-	ciphertext_page = fscrypt_alloc_bounce_page(ctx, gfp_flags);
-	if (IS_ERR(ciphertext_page))
-		goto errout;
+	ciphertext_page = fscrypt_alloc_bounce_page(gfp_flags);
+	if (!ciphertext_page)
+		return ERR_PTR(-ENOMEM);
 
-	ctx->w.control_page = page;
 	err = fscrypt_do_page_crypto(inode, FS_ENCRYPT, lblk_num,
 				     page, ciphertext_page, len, offs,
 				     gfp_flags);
 	if (err) {
-		ciphertext_page = ERR_PTR(err);
-		goto errout;
+		fscrypt_free_bounce_page(ciphertext_page);
+		return ERR_PTR(err);
 	}
 	SetPagePrivate(ciphertext_page);
-	set_page_private(ciphertext_page, (unsigned long)ctx);
-	lock_page(ciphertext_page);
-	return ciphertext_page;
-
-errout:
-	fscrypt_release_ctx(ctx);
+	set_page_private(ciphertext_page, (unsigned long)page);
 	return ciphertext_page;
 }
 EXPORT_SYMBOL(fscrypt_encrypt_page);
@@ -355,18 +331,6 @@ const struct dentry_operations fscrypt_d_ops = {
 	.d_revalidate = fscrypt_d_revalidate,
 };
 
-void fscrypt_restore_control_page(struct page *page)
-{
-	struct fscrypt_ctx *ctx;
-
-	ctx = (struct fscrypt_ctx *)page_private(page);
-	set_page_private(page, (unsigned long)NULL);
-	ClearPagePrivate(page);
-	unlock_page(page);
-	fscrypt_release_ctx(ctx);
-}
-EXPORT_SYMBOL(fscrypt_restore_control_page);
-
 static void fscrypt_destroy(void)
 {
 	struct fscrypt_ctx *pos, *n;
diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
index 7da276159593..4122ee1a0b7b 100644
--- a/fs/crypto/fscrypt_private.h
+++ b/fs/crypto/fscrypt_private.h
@@ -94,7 +94,6 @@ typedef enum {
 } fscrypt_direction_t;
 
 #define FS_CTX_REQUIRES_FREE_ENCRYPT_FL		0x00000001
-#define FS_CTX_HAS_BOUNCE_BUFFER_FL		0x00000002
 
 static inline bool fscrypt_valid_enc_modes(u32 contents_mode,
 					   u32 filenames_mode)
@@ -123,8 +122,7 @@ extern int fscrypt_do_page_crypto(const struct inode *inode,
 				  struct page *dest_page,
 				  unsigned int len, unsigned int offs,
 				  gfp_t gfp_flags);
-extern struct page *fscrypt_alloc_bounce_page(struct fscrypt_ctx *ctx,
-					      gfp_t gfp_flags);
+extern struct page *fscrypt_alloc_bounce_page(gfp_t gfp_flags);
 extern const struct dentry_operations fscrypt_d_ops;
 
 extern void __printf(3, 4) __cold