[virtiofsd PATCH v4 3/4] virtiofsd: support per-file DAX negotiation in FUSE_INIT

@@ -372,6 +372,11 @@ struct fuse_file_info {
  */
 #define FUSE_CAP_HANDLE_KILLPRIV_V2 (1 << 28)
 
+/**
+ * Indicates support for per-file DAX.
+ */
+#define FUSE_CAP_PERFILE_DAX (1 << 29)
+
 /**
  * Ioctl flags
  *

@@ -2065,6 +2065,9 @@ static void do_init(fuse_req_t req, fuse_ino_t nodeid,
     if (arg->flags & FUSE_HANDLE_KILLPRIV_V2) {
         se->conn.capable |= FUSE_CAP_HANDLE_KILLPRIV_V2;
     }
+    if (arg->flags & FUSE_PERFILE_DAX) {
+        se->conn.capable |= FUSE_CAP_PERFILE_DAX;
+    }
 #ifdef HAVE_SPLICE
 #ifdef HAVE_VMSPLICE
     se->conn.capable |= FUSE_CAP_SPLICE_WRITE | FUSE_CAP_SPLICE_MOVE;

@@ -2180,6 +2183,9 @@ static void do_init(fuse_req_t req, fuse_ino_t nodeid,
     if (se->conn.want & FUSE_CAP_POSIX_ACL) {
         outarg.flags |= FUSE_POSIX_ACL;
     }
+    if (se->op.ioctl && (se->conn.want & FUSE_CAP_PERFILE_DAX)) {
+        outarg.flags |= FUSE_PERFILE_DAX;
+    }
     outarg.max_readahead = se->conn.max_readahead;
     outarg.max_write = se->conn.max_write;
     if (se->conn.max_background >= (1 << 16)) {

@@ -53,8 +53,10 @@
 #include <sys/syscall.h>
 #include <sys/wait.h>
 #include <sys/xattr.h>
+#include <sys/vfs.h>
 #include <syslog.h>
 #include <linux/fs.h>
+#include <linux/magic.h>
 
 #include "qemu/cutils.h"
 #include "passthrough_helpers.h"

@@ -136,6 +138,13 @@ enum {
     SANDBOX_CHROOT,
 };
 
+/* capability of storing DAX flag persistently */
+enum {
+    DAX_CAP_NONE,  /* not supported */
+    DAX_CAP_FLAGS, /* stored in flags (FS_IOC_GETFLAGS/FS_IOC_SETFLAGS) */
+    DAX_CAP_XATTR, /* stored in xflags (FS_IOC_FSGETXATTR/FS_IOC_FSSETXATTR) */
+};
+
 typedef struct xattr_map_entry {
     char *key;
     char *prepend;

@@ -161,6 +170,7 @@ struct lo_data {
     int readdirplus_clear;
     int allow_direct_io;
     int announce_submounts;
+    int perfile_dax_cap; /* capability of backend fs */
     bool use_statx;
     struct lo_inode root;
     GHashTable *inodes; /* protected by lo->mutex */

@@ -703,6 +713,10 @@ static void lo_init(void *userdata, struct fuse_conn_info *conn)
         conn->want &= ~FUSE_CAP_HANDLE_KILLPRIV_V2;
         lo->killpriv_v2 = 0;
     }
+
+    if (conn->capable & FUSE_CAP_PERFILE_DAX && lo->perfile_dax_cap ) {
+        conn->want |= FUSE_CAP_PERFILE_DAX;
+    }
 }
 
 static void lo_getattr(fuse_req_t req, fuse_ino_t ino,

@@ -3800,6 +3814,7 @@ static void setup_root(struct lo_data *lo, struct lo_inode *root)
     int fd, res;
     struct stat stat;
     uint64_t mnt_id;
+    struct statfs statfs;
 
     fd = open("/", O_PATH);
     if (fd == -1) {

@@ -3826,6 +3841,20 @@ static void setup_root(struct lo_data *lo, struct lo_inode *root)
         root->posix_locks = g_hash_table_new_full(
             g_direct_hash, g_direct_equal, NULL, posix_locks_value_destroy);
     }
+
+    /*
+     * Currently only ext4/xfs since linux kernel v5.8 support storing
+     * FS_DAX_FL flag persistently. Ext4 accesses this flag through
+     * FS_IOC_G[S]ETFLAGS ioctl, while xfs accesses this flag through
+     * FS_IOC_FSG[S]ETXATTR ioctl.
+     */
+    res = fstatfs(fd, &statfs);
+    if (!res) {
+	if (statfs.f_type == EXT4_SUPER_MAGIC)
+	    lo->perfile_dax_cap = DAX_CAP_FLAGS;
+	else if (statfs.f_type == XFS_SUPER_MAGIC)
+	    lo->perfile_dax_cap = DAX_CAP_XATTR;
+    }
 }
 
 static guint lo_key_hash(gconstpointer key)