erofs: support STATX_DIOALIGN

Add support for STATX_DIOALIGN to EROFS, so that direct I/O
alignment restrictions are exposed to userspace in a generic
way.

[Before]
```
./statx_test /mnt/erofs/testfile
statx(/mnt/erofs/testfile) = 0
dio mem align:0
dio offset align:0
```

[After]
```
./statx_test /mnt/erofs/testfile
statx(/mnt/erofs/testfile) = 0
dio mem align:512
dio offset align:512
```

Signed-off-by: Hongbo Li <[email protected]>
Reviewed-by: Chao Yu <[email protected]>
Signed-off-by: Gao Xiang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

fs/erofs/inode.c

@@ -334,14 +334,29 @@ int erofs_getattr(struct mnt_idmap *idmap, const struct path *path,
 		  unsigned int query_flags)
 {
 	struct inode *const inode = d_inode(path->dentry);
+	bool compressed =
+		erofs_inode_is_data_compressed(EROFS_I(inode)->datalayout);
 
-	if (erofs_inode_is_data_compressed(EROFS_I(inode)->datalayout))
+	if (compressed)
 		stat->attributes |= STATX_ATTR_COMPRESSED;
-
 	stat->attributes |= STATX_ATTR_IMMUTABLE;
 	stat->attributes_mask |= (STATX_ATTR_COMPRESSED |
 				  STATX_ATTR_IMMUTABLE);
 
+	/*
+	 * Return the DIO alignment restrictions if requested.
+	 *
+	 * In EROFS, STATX_DIOALIGN is not supported in ondemand mode and
+	 * compressed files, so in these cases we report no DIO support.
+	 */
+	if ((request_mask & STATX_DIOALIGN) && S_ISREG(inode->i_mode)) {
+		stat->result_mask |= STATX_DIOALIGN;
+		if (!erofs_is_fscache_mode(inode->i_sb) && !compressed) {
+			stat->dio_mem_align =
+				bdev_logical_block_size(inode->i_sb->s_bdev);
+			stat->dio_offset_align = stat->dio_mem_align;
+		}
+	}
 	generic_fillattr(idmap, request_mask, inode, stat);
 	return 0;
 }