[Virtio-fs] [PATCH v6 0/7] fuse,virtiofs: support per-file DAX

[JeffleXu]

On 10/18/21 11:21 PM, Vivek Goyal wrote:
> On Mon, Oct 11, 2021 at 11:00:45AM +0800, Jeffle Xu wrote:
>> changes since v5:
>> Overall Design Changes:
>> 1. virtiofsd now supports ioctl (only FS_IOC_SETFLAGS and
>>   FS_IOC_FSSETXATTR), so that users inside guest could set/clear
>>   persistent inode flags now. (FUSE kernel module has already supported
>>   .ioctl(), virtiofsd need to suuport it.)
> 
> So no changes needed in fuse side (kernel) to support FS_IOC_FSSETXATTR?
> Only virtiofsd needs to be changed. That sounds good.
> 

Yes, the fuse kernel modules has already supported FUSE_IOCTL.

Per inode DAX on ext4/xfs will also call d_mark_dontcache() and try to
evict this inode as soon as possible when the persistent (DAX) inode
attribute has changed, just like [1].

But because of following reason:
> 
>> 2. The
>>   algorithm used by virtiofsd to determine whether DAX shall be enabled
>>   or not is totally implementation specific, and thus the following
>>   scenario may exist: users inside guest has already set related persistent
>>   inode flag (i.e. FS_XFLAG_DAX) on corresponding file but FUSE server finnaly
>>   decides not to enable DAX for this file.

If we always call d_mark_dontcache() and try to evict this inode when
the persistent (DAX) inode attribute has changed, the DAX state returned
by virtiofsd may sustain the same, and thus the previous eviction is
totally wasted and unnecessary.

So, as the following said,

>> Also because of this, d_mark_dontcache() is
>>   not called when FS_IOC_SETFLAGS/FS_IOC_FSSETXATTR ioctl is done inside
>>   guest. It's delayed to be done if the FUSE_ATTR_DAX flag **indeed**
>>   changes (as showed in patch 6).

the call for d_mark_dontcache() and inode eviction is delayed when the
DAX state returned by virtiofsd **indeed** changed (when dentry is timed
out and a new FUSE_LOOKUP is requested). But the defect is that, if '-o
cache=always' is set for virtiofsd, then the DAX state won't be updated
for a long time, after users have changed the persistent (DAX) inode
attribute inside guest via FS_IOC_FSSETXATTR ioctl.



[1] https://www.spinics.net/lists/linux-fsdevel/msg200851.html

-- 
Thanks,
Jeffle