Support for longnames for files/directories (Linux part)

This patch adds the ability for zfs to support file/dir name up to 1023
bytes. This number is chosen so we can support up to 255 4-byte
characters. This new feature is represented by the new feature flag
feature@longname.

A new dataset property "longname" is also introduced to toggle longname
support for each dataset individually. This property can be disabled,
even if it contains longname files. In such case, new file cannot be
created with longname but existing longname files can still be looked
up.

Note that, to my knowledge native Linux filesystems don't support name
longer than 255 bytes. So there might be programs not able to work with
longname.

Note that NFS server may needs to use exportfs_get_name to reconnect
dentries, and the buffer being passed is limit to NAME_MAX+1 (256). So
NFS may not work when longname is enabled.

Note, FreeBSD vfs layer imposes a limit of 255 name lengh, so even
though we add code to support it here, it won't actually work.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Chunwei Chen <david.chen@nutanix.com>
Closes #15921

include/sys/fs/zfs.h

@@ -81,6 +81,7 @@ typedef enum dmu_objset_type {
  * All of these include the terminating NUL byte.
  */
 #define	ZAP_MAXNAMELEN 256
+#define	ZAP_MAXNAMELEN_NEW 1024
 #define	ZAP_MAXVALUELEN (1024 * 8)
 #define	ZAP_OLDMAXVALUELEN 1024
 #define	ZFS_MAX_DATASET_NAME_LEN 256
@@ -194,6 +195,7 @@ typedef enum {
 	ZFS_PROP_PREFETCH,
 	ZFS_PROP_VOLTHREADING,
 	ZFS_PROP_DIRECT,
+	ZFS_PROP_LONGNAME,
 	ZFS_NUM_PROPS
 } zfs_prop_t;
 

include/sys/zap.h

@@ -315,7 +315,7 @@ int zap_count(objset_t *ds, uint64_t zapobj, uint64_t *count);
  * match must be exact (ie, same as mask=-1ULL).
  */
 int zap_value_search(objset_t *os, uint64_t zapobj,
-    uint64_t value, uint64_t mask, char *name);
+    uint64_t value, uint64_t mask, char *name, uint64_t namelen);
 
 /*
  * Transfer all the entries from fromobj into intoobj.  Only works on
@@ -387,6 +387,7 @@ void zap_fini(void);
  * Alloc and free zap_attribute_t.
  */
 zap_attribute_t *zap_attribute_alloc(void);
+zap_attribute_t *zap_attribute_long_alloc(void);
 void zap_attribute_free(zap_attribute_t *attrp);
 
 /*

include/sys/zfs_ioctl.h

@@ -143,7 +143,7 @@ typedef enum drr_headertype {
  * default use of "zfs send" won't encounter the bug mentioned above.
  */
 #define	DMU_BACKUP_FEATURE_SWITCH_TO_LARGE_BLOCKS (1 << 27)
-/* flag #28 is reserved for a Nutanix feature */
+#define	DMU_BACKUP_FEATURE_LONGNAME		(1 << 28)
 
 /*
  * Mask of all supported backup features
@@ -154,7 +154,7 @@ typedef enum drr_headertype {
     DMU_BACKUP_FEATURE_COMPRESSED | DMU_BACKUP_FEATURE_LARGE_DNODE | \
     DMU_BACKUP_FEATURE_RAW | DMU_BACKUP_FEATURE_HOLDS | \
     DMU_BACKUP_FEATURE_REDACTED | DMU_BACKUP_FEATURE_SWITCH_TO_LARGE_BLOCKS | \
-    DMU_BACKUP_FEATURE_ZSTD)
+    DMU_BACKUP_FEATURE_ZSTD | DMU_BACKUP_FEATURE_LONGNAME)
 
 /* Are all features in the given flag word currently supported? */
 #define	DMU_STREAM_SUPPORTED(x)	(!((x) & ~DMU_BACKUP_FEATURE_MASK))