On 7/20/21 2:41 AM, Vivek Goyal wrote:
> On Fri, Jul 16, 2021 at 06:47:52PM +0800, Jeffle Xu wrote:
>> 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 <jefflexu@xxxxxxxxxxxxxxxxx>
>
> [..]
>> 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@xxxxxxxxxxxxxxxxx/T/#m3f3407158b2c028694c85d82d0d6bd0387f4e24e
>
> 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
