Re: [PATCH v2 3/4] fuse: add per-file DAX flag

On 7/20/21 2:41 AM, Vivek Goyal wrote:

On Fri, Jul 16, 2021 at 06:47:52PM +0800, Jeffle Xu wrote:

quoted

Add one flag for fuse_attr.flags indicating if DAX shall be enabled for
this file.

When the per-file DAX flag changes for an *opened* file, the state of
the file won't be updated until this file is closed and reopened later.

Signed-off-by: Jeffle Xu <redacted>

[..]

quoted

diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index 36ed092227fa..90c9df10d37a 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h

@@ -184,6 +184,9 @@
  *
  *  7.34
  *  - add FUSE_SYNCFS
+ *
+ *  7.35
+ *  - add FUSE_ATTR_DAX
  */
 
 #ifndef _LINUX_FUSE_H

@@ -449,8 +452,10 @@ struct fuse_file_lock {
  * fuse_attr flags
  *
  * FUSE_ATTR_SUBMOUNT: Object is a submount root
+ * FUSE_ATTR_DAX: Enable DAX for this file in per-file DAX mode
  */
 #define FUSE_ATTR_SUBMOUNT      (1 << 0)
+#define FUSE_ATTR_DAX		(1 << 1)

Generic fuse changes (addition of FUSE_ATTR_DAX) should probably in
a separate patch. 

Got it.

I am not clear on one thing. If we are planning to rely on persistent
inode attr (FS_XFLAG_DAX as per Documentation/filesystems/dax.rst), then
why fuse server needs to communicate the state of that attr using a 
flag? Can client directly query it?  I am not sure where at these
attrs stored and if fuse protocol currently supports it.

There are two issues.

1. FUSE server side: Algorithm of deciding whether DAX is enabled for a
file.

As I previously replied in [1], FUSE server must enable DAX if the
backend file is flagged with FS_XFLAG_DAX, to make the FS_XFLAG_DAX
previously set by FUSE client effective.

But I will argue that FUSE server also has the flexibility of the
algorithm implementation. Even if guest queries FS_XFLAG_DAX by
GETFLAGS/FSGETXATTR ioctl, FUSE server can still enable DAX when the
backend file is not FS_XFLAG_DAX flagged.


2. The protocol between server and client.

extending LOOKUP vs. LOOKUP + GETFLAGS/FSGETXATTR ioctl

As I said in [1], client can directly query the FS_XFLAG_DAX flag, but
there will be one more round trip.


[1]
https://lore.kernel.org/linux-fsdevel/031efb1d-7c0d-35fb-c147-dcc3b6cac0ef@linux.alibaba.com/T/#m3f3407158b2c028694c85d82d0d6bd0387f4e24e (local)

What about flag STATX_ATTR_DAX. We probably should report that too
in stat if we are using dax on the inode?

VFS will automatically report STATX_ATTR_DAX if inode is in DAX mode,
e.g., in vfs_getattr_nosec().



-- 
Thanks,
Jeffle
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