fscrypt: make the bounce page pool opt-in instead of opt-out

Replace FS_CFLG_OWN_PAGES with a bit flag 'needs_bounce_pages' which has
the opposite meaning.  I.e., filesystems now opt into the bounce page
pool instead of opt out.  Make fscrypt_alloc_bounce_page() check that
the bounce page pool has been initialized.

I believe the opt-in makes more sense, since nothing else in
fscrypt_operations is opt-out, and these days filesystems can choose to
use blk-crypto which doesn't need the fscrypt bounce page pool.  Also, I
happen to be planning to add two more flags, and I wanted to fix the
"FS_CFLG_" name anyway as it wasn't prefixed with "FSCRYPT_".

Link: https://lore.kernel.org/r/20230925055451.59499-3-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

1 files changed, 11 insertions, 9 deletions

diff --git a/include/linux/fscrypt.h b/include/linux/fscrypt.h
index b0037566ce30..4505078e89b7 100644
--- a/include/linux/fscrypt.h
+++ b/include/linux/fscrypt.h
@@ -59,18 +59,20 @@ struct fscrypt_name {
 
 #ifdef CONFIG_FS_ENCRYPTION
 
-/*
- * If set, the fscrypt bounce page pool won't be allocated (unless another
- * filesystem needs it).  Set this if the filesystem always uses its own bounce
- * pages for writes and therefore won't need the fscrypt bounce page pool.
- */
-#define FS_CFLG_OWN_PAGES (1U << 1)
-
 /* Crypto operations for filesystems */
 struct fscrypt_operations {
 
-	/* Set of optional flags; see above for allowed flags */
-	unsigned int flags;
+	/*
+	 * If set, then fs/crypto/ will allocate a global bounce page pool the
+	 * first time an encryption key is set up for a file.  The bounce page
+	 * pool is required by the following functions:
+	 *
+	 * - fscrypt_encrypt_pagecache_blocks()
+	 * - fscrypt_zeroout_range() for files not using inline crypto
+	 *
+	 * If the filesystem doesn't use those, it doesn't need to set this.
+	 */
+	unsigned int needs_bounce_pages : 1;
 
 	/*
 	 * This field exists only for backwards compatibility reasons and should