[virtiofsd PATCH v4 4/4] virtiofsd: support per-file DAX in FUSE_LOOKUP

[Jeffle Xu <jefflexu@xxxxxxxxxxxxxxxxx>]

For passthrough, when the corresponding virtiofs in guest is mounted
with '-o dax=inode', advertise that the file is capable of per-file
DAX if the inode in the backend fs is marked with FS_DAX_FL flag.

Signed-off-by: Jeffle Xu <jefflexu@xxxxxxxxxxxxxxxxx>
---
 tools/virtiofsd/passthrough_ll.c | 43 ++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
index 5b6228210f..4cbd904248 100644
--- a/tools/virtiofsd/passthrough_ll.c
+++ b/tools/virtiofsd/passthrough_ll.c
@@ -171,6 +171,7 @@ struct lo_data {
     int allow_direct_io;
     int announce_submounts;
     int perfile_dax_cap; /* capability of backend fs */
+    bool perfile_dax; /* enable per-file DAX or not */
     bool use_statx;
     struct lo_inode root;
     GHashTable *inodes; /* protected by lo->mutex */
@@ -716,6 +717,10 @@ static void lo_init(void *userdata, struct fuse_conn_info *conn)
 
     if (conn->capable & FUSE_CAP_PERFILE_DAX && lo->perfile_dax_cap ) {
         conn->want |= FUSE_CAP_PERFILE_DAX;
+	lo->perfile_dax = 1;
+    }
+    else {
+	lo->perfile_dax = 0;
     }
 }
 
@@ -983,6 +988,41 @@ static int do_statx(struct lo_data *lo, int dirfd, const char *pathname,
     return 0;
 }
 
+/*
+ * If the file is marked with FS_DAX_FL or FS_XFLAG_DAX, then DAX should be
+ * enabled for this file.
+ */
+static bool lo_should_enable_dax(struct lo_data *lo, struct lo_inode *dir,
+				 const char *name)
+{
+    int res, fd;
+    int ret = false;;
+    unsigned int attr;
+    struct fsxattr xattr;
+
+    if (!lo->perfile_dax)
+	return false;
+
+    /* Open file without O_PATH, so that ioctl can be called. */
+    fd = openat(dir->fd, name, O_NOFOLLOW);
+    if (fd == -1)
+        return false;
+
+    if (lo->perfile_dax_cap == DAX_CAP_FLAGS) {
+        res = ioctl(fd, FS_IOC_GETFLAGS, &attr);
+        if (!res && (attr & FS_DAX_FL))
+	    ret = true;
+    }
+    else if (lo->perfile_dax_cap == DAX_CAP_XATTR) {
+	res = ioctl(fd, FS_IOC_FSGETXATTR, &xattr);
+	if (!res && (xattr.fsx_xflags & FS_XFLAG_DAX))
+	    ret = true;
+    }
+
+    close(fd);
+    return ret;
+}
+
 /*
  * Increments nlookup on the inode on success. unref_inode_lolocked() must be
  * called eventually to decrement nlookup again. If inodep is non-NULL, the
@@ -1038,6 +1078,9 @@ static int lo_do_lookup(fuse_req_t req, fuse_ino_t parent, const char *name,
         e->attr_flags |= FUSE_ATTR_SUBMOUNT;
     }
 
+    if (lo_should_enable_dax(lo, dir, name))
+	e->attr_flags |= FUSE_ATTR_DAX;
+
     inode = lo_find(lo, &e->attr, mnt_id);
     if (inode) {
         close(newfd);
-- 
2.27.0