Implement statx(STATX_DIOALIGN) so applications can discover correct O_DIRECT alignment

include/sys/zfs_vnops.h

@@ -20,6 +20,7 @@
  */
 /*
  * Copyright (c) 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2025, Rob Norris <[email protected]>
  */
 
 #ifndef	_SYS_FS_ZFS_VNOPS_H
@@ -42,6 +43,8 @@ extern int zfs_clone_range_replay(znode_t *, uint64_t, uint64_t, uint64_t,
 extern int zfs_getsecattr(znode_t *, vsecattr_t *, int, cred_t *);
 extern int zfs_setsecattr(znode_t *, vsecattr_t *, int, cred_t *);
 
+extern int zfs_get_direct_alignment(znode_t *, uint64_t *);
+
 extern int mappedread(znode_t *, int, zfs_uio_t *);
 extern int mappedread_sf(znode_t *, int, zfs_uio_t *);
 extern void update_pages(znode_t *, int64_t, int, objset_t *);

module/os/linux/zfs/zpl_inode.c

@@ -21,6 +21,7 @@
 /*
  * Copyright (c) 2011, Lawrence Livermore National Security, LLC.
  * Copyright (c) 2015 by Chunwei Chen. All rights reserved.
+ * Copyright (c) 2025, Rob Norris <[email protected]>
  */
 
 
@@ -30,6 +31,7 @@
 #include <sys/zfs_vnops.h>
 #include <sys/zfs_znode.h>
 #include <sys/dmu_objset.h>
+#include <sys/spa_impl.h>
 #include <sys/vfs.h>
 #include <sys/zpl.h>
 #include <sys/file.h>
@@ -490,6 +492,17 @@ zpl_getattr_impl(const struct path *path, struct kstat *stat, u32 request_mask,
 	}
 #endif
 
+#ifdef STATX_DIOALIGN
+	if (request_mask & STATX_DIOALIGN) {
+		uint64_t align;
+		if (zfs_get_direct_alignment(zp, &align) == 0) {
+			stat->dio_mem_align = PAGE_SIZE;
+			stat->dio_offset_align = align;
+			stat->result_mask |= STATX_DIOALIGN;
+		}
+	}
+#endif
+
 #ifdef STATX_ATTR_IMMUTABLE
 	if (zp->z_pflags & ZFS_IMMUTABLE)
 		stat->attributes |= STATX_ATTR_IMMUTABLE;

module/zfs/zfs_vnops.c

@@ -25,6 +25,7 @@
  * Copyright (c) 2015 by Chunwei Chen. All rights reserved.
  * Copyright 2017 Nexenta Systems, Inc.
  * Copyright (c) 2021, 2022 by Pawel Jakub Dawidek
+ * Copyright (c) 2025, Rob Norris <[email protected]>
  */
 
 /* Portions Copyright 2007 Jeremy Teo */
@@ -1083,6 +1084,44 @@ zfs_setsecattr(znode_t *zp, vsecattr_t *vsecp, int flag, cred_t *cr)
 	return (error);
 }
 
+/*
+ * Get the optimal alignment to ensure direct IO can be performed without
+ * incurring any RMW penalty on write. If direct IO is not enabled for this
+ * file, returns an error.
+ */
+int
+zfs_get_direct_alignment(znode_t *zp, uint64_t *alignp)
+{
+	zfsvfs_t *zfsvfs = ZTOZSB(zp);
+
+	if (!zfs_dio_enabled || zfsvfs->z_os->os_direct == ZFS_DIRECT_DISABLED)
+		return (SET_ERROR(EOPNOTSUPP));
+
+	/*
+	 * If the file has multiple blocks, then its block size is fixed
+	 * forever, and so is the ideal alignment.
+	 *
+	 * If however it only has a single block, then we want to return the
+	 * max block size it could possibly grown to (ie, the dataset
+	 * recordsize). We do this so that a program querying alignment
+	 * immediately after the file is created gets a value that won't change
+	 * once the file has grown into the second block and beyond.
+	 *
+	 * Because we don't have a count of blocks easily available here, we
+	 * check if the apparent file size is smaller than its current block
+	 * size (meaning, the file hasn't yet grown into the current block
+	 * size) and then, check if the block size is smaller than the dataset
+	 * maximum (meaning, if the file grew past the current block size, the
+	 * block size could would be increased).
+	 */
+	if (zp->z_size <= zp->z_blksz && zp->z_blksz < zfsvfs->z_max_blksz)
+		*alignp = MAX(zfsvfs->z_max_blksz, PAGE_SIZE);
+	else
+		*alignp = MAX(zp->z_blksz, PAGE_SIZE);
+
+	return (0);
+}
+
 #ifdef ZFS_DEBUG
 static int zil_fault_io = 0;
 #endif

tests/runfiles/common.run

@@ -981,7 +981,7 @@ tests = ['sparse_001_pos']
 tags = ['functional', 'sparse']
 
 [tests/functional/stat]
-tests = ['stat_001_pos']
+tests = ['stat_001_pos', 'statx_dioalign']
 tags = ['functional', 'stat']
 
 [tests/functional/suid]

tests/test-runner/bin/zts-report.py.in

@@ -128,6 +128,11 @@ idmap_reason = 'Idmapped mount needs kernel 5.12+'
 #
 cfr_reason = 'Kernel copy_file_range support required'
 
+#
+# Some statx fields are not supported by all kernels
+#
+statx_reason = 'Needed statx(2) field not supported on this kernel'
+
 if sys.platform.startswith('freebsd'):
     cfr_cross_reason = 'copy_file_range(2) cross-filesystem needs FreeBSD 14+'
 else:
@@ -293,7 +298,8 @@ if sys.platform.startswith('freebsd'):
         'block_cloning/block_cloning_cross_enc_dataset':
             ['SKIP', cfr_cross_reason],
         'block_cloning/block_cloning_copyfilerange_cross_dataset':
-            ['SKIP', cfr_cross_reason]
+            ['SKIP', cfr_cross_reason],
+        'stat/statx_dioalign': ['SKIP', 'na_reason'],
     })
 elif sys.platform.startswith('linux'):
     maybe.update({
@@ -361,6 +367,7 @@ elif sys.platform.startswith('linux'):
         'mmp/mmp_active_import': ['FAIL', known_reason],
         'mmp/mmp_exported_import': ['FAIL', known_reason],
         'mmp/mmp_inactive_import': ['FAIL', known_reason],
+        'stat/statx_dioalign': ['SKIP', 'statx_reason'],
     })
 
 

tests/zfs-tests/cmd/.gitignore

@@ -36,6 +36,7 @@
 /rename_dir
 /rm_lnkcnt_zero_file
 /send_doall
+/statx
 /stride_dd
 /threadsappend
 /user_ns_exec
@@ -54,3 +55,4 @@
 /skein_test
 /sha2_test
 /idmap_util
+/statx

tests/zfs-tests/cmd/Makefile.am

@@ -126,6 +126,7 @@ if BUILD_LINUX
 scripts_zfs_tests_bin_PROGRAMS += %D%/getversion
 scripts_zfs_tests_bin_PROGRAMS += %D%/user_ns_exec
 scripts_zfs_tests_bin_PROGRAMS += %D%/renameat2
+scripts_zfs_tests_bin_PROGRAMS += %D%/statx
 scripts_zfs_tests_bin_PROGRAMS += %D%/xattrtest
 scripts_zfs_tests_bin_PROGRAMS += %D%/zed_fd_spill-zedlet
 scripts_zfs_tests_bin_PROGRAMS += %D%/idmap_util