[PATCH RFC 8/8] ext4: Add a per-file provision override xattr

[Sarthak Kukreti <sarthakkukreti@chromium.org>]

From: Sarthak Kukreti <sarthakkukreti@chromium.org>

Adds a per-file provision override that allows select files to
override the per-mount setting for provisioning blocks on allocation.

This acts as a mechanism to allow mounts using provision to
replicate the current behavior for fallocate() and only preserve
space at the filesystem level.

Signed-off-by: Sarthak Kukreti <sarthakkukreti@chromium.org>
---
 fs/ext4/extents.c | 32 ++++++++++++++++++++++++++++++++
 fs/ext4/xattr.h   |  1 +
 2 files changed, 33 insertions(+)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 746213b5ec3d..a9ed908b2ebe 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -4424,6 +4424,26 @@ int ext4_ext_truncate(handle_t *handle, struct inode *inode)
 	return err;
 }
 
+int ext4_provision_support(struct inode *inode)
+{
+	char provision;
+	int ret =
+		ext4_xattr_get(inode, EXT4_XATTR_INDEX_TRUSTED,
+			       EXT4_XATTR_NAME_PROVISION_POLICY, &provision, 1);
+
+	if (ret < 0)
+		return ret;
+
+	switch (provision) {
+	case 'y':
+		return 1;
+	case 'n':
+		return 0;
+	default:
+		return -EINVAL;
+	}
+}
+
 static int ext4_alloc_file_blocks(struct file *file, ext4_lblk_t offset,
 				  ext4_lblk_t len, loff_t new_size,
 				  int flags)
@@ -4436,12 +4456,24 @@ static int ext4_alloc_file_blocks(struct file *file, ext4_lblk_t offset,
 	struct ext4_map_blocks map;
 	unsigned int credits;
 	loff_t epos;
+	bool provision = false;
+	int file_provision_override = -1;
 
 	/*
 	 * Attempt to provision file blocks if the mount is mounted with
 	 * provision.
 	 */
 	if (test_opt2(inode->i_sb, PROVISION))
+		provision = true;
+
+	/*
+	 * Use file-specific override, if available.
+	 */
+	file_provision_override = ext4_provision_support(inode);
+	if (file_provision_override >= 0)
+		provision &= file_provision_override;
+
+	if (provision)
 		flags |= EXT4_GET_BLOCKS_PROVISION;
 
 	BUG_ON(!ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS));
diff --git a/fs/ext4/xattr.h b/fs/ext4/xattr.h
index 824faf0b15a8..69e97f853b0c 100644
--- a/fs/ext4/xattr.h
+++ b/fs/ext4/xattr.h
@@ -140,6 +140,7 @@ extern const struct xattr_handler ext4_xattr_security_handler;
 extern const struct xattr_handler ext4_xattr_hurd_handler;
 
 #define EXT4_XATTR_NAME_ENCRYPTION_CONTEXT "c"
+#define EXT4_XATTR_NAME_PROVISION_POLICY "provision"
 
 /*
  * The EXT4_STATE_NO_EXPAND is overloaded and used for two purposes.
-- 
2.31.0