f2fs: support STATX_DIOALIGN

Add support for STATX_DIOALIGN to f2fs, so that direct I/O alignment
restrictions are exposed to userspace in a generic way.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Jaegeuk Kim <jaegeuk@kernel.org>
Link: https://lore.kernel.org/r/20220827065851.135710-8-ebiggers@kernel.org

fs/f2fs/file.c

@@ -847,6 +847,24 @@ int f2fs_getattr(struct user_namespace *mnt_userns, const struct path *path,
 		stat->btime.tv_nsec = fi->i_crtime.tv_nsec;
 	}
 
+	/*
+	 * Return the DIO alignment restrictions if requested.  We only return
+	 * this information when requested, since on encrypted files it might
+	 * take a fair bit of work to get if the file wasn't opened recently.
+	 *
+	 * f2fs sometimes supports DIO reads but not DIO writes.  STATX_DIOALIGN
+	 * cannot represent that, so in that case we report no DIO support.
+	 */
+	if ((request_mask & STATX_DIOALIGN) && S_ISREG(inode->i_mode)) {
+		unsigned int bsize = i_blocksize(inode);
+
+		stat->result_mask |= STATX_DIOALIGN;
+		if (!f2fs_force_buffered_io(inode, WRITE)) {
+			stat->dio_mem_align = bsize;
+			stat->dio_offset_align = bsize;
+		}
+	}
+
 	flags = fi->i_flags;
 	if (flags & F2FS_COMPR_FL)
 		stat->attributes |= STATX_ATTR_COMPRESSED;