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

On 8/18/21 3:00 AM, Dr. David Alan Gilbert wrote:

* Jeffle Xu (jefflexu@linux.alibaba.com) wrote:

quoted

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 <redacted>
---
 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;

Doesn't that defeat the whole benefit of using O_PATH - i.e. that we
might stumble into a /dev node or something else we're not allowed to
open?

As far as I know, virtiofsd will pivot_root/chroot to the source
directory, and can only access files inside the source directory
specified by "-o source=". Then where do these unexpected files come
from? Besides, fd opened without O_PATH here is temporary and used for
FS_IOC_GETFLAGS/FS_IOC_FSGETXATTR ioctl only. It's closed when the
function returns.

quoted

+    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;
+    }

This all looks pretty expensive for each lookup.

Yes. it can be somehow optimized if we can agree on the way of storing
the dax flag persistently.

-- 
Thanks,
Jeffle
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization