[Virtio-fs] [PATCH v7 5/6] virtiofsd: implement xflag based dax policy

[JeffleXu]

On 12/10/21 6:02 AM, Vivek Goyal wrote:
> On Tue, Nov 02, 2021 at 01:56:45PM +0800, Jeffle Xu wrote:
>> The per inode DAX feature in ext4/xfs uses the persistent inode flag,
>> i.e. FS_DAX_FL/FS_XFLAG_DAX, to indicate if DAX shall be enabled for
>> this file or not when filesystem is mounted in per inode DAX mode.
>>
>> To keep compatible with this feature in ext4/xfs, virtiofs also supports
>> enabling DAX depending on the persistent inode flag, which may be
>> set/cleared by users inside guest or admin on host.
>>
>> This policy is used when '-o dax=inode' option is specified for
>> virtiofsd.
>>
>> Signed-off-by: Jeffle Xu <jefflexu at linux.alibaba.com>
>> ---
>>  tools/virtiofsd/passthrough_ll.c | 33 ++++++++++++++++++++++++++++++++
>>  1 file changed, 33 insertions(+)
>>
>> diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
>> index 999c906da2..dac5063594 100644
>> --- a/tools/virtiofsd/passthrough_ll.c
>> +++ b/tools/virtiofsd/passthrough_ll.c
>> @@ -1026,6 +1026,35 @@ static int do_statx(struct lo_data *lo, int dirfd, const char *pathname,
>>      return 0;
>>  }
>>  
>> +static bool lo_should_enable_dax(struct lo_data *lo, struct lo_inode *inode,
>> +                                 struct fuse_entry_param *e)
>> +{
>> +    if (lo->dax == INODE_DAX_NONE || !S_ISREG(e->attr.st_mode)) {
>> +        return false;
>> +    }
>> +
>> +    if (lo->dax == INODE_DAX_INODE) {
>> +        int res, fd;
>> +        int ret = false;
>> +        unsigned int attr;
>> +
>> +        fd = lo_inode_open(lo, inode, O_RDONLY);
>> +        if (fd == -1) {
>> +            return false;
>> +        }
>> +
>> +        res = ioctl(fd, FS_IOC_GETFLAGS, &attr);
>> +        if (!res && (attr & FS_DAX_FL)) {
>> +            ret = true;
>> +        }
>> +
>> +        close(fd);
>> +        return ret;
>> +    }
>> +
>> +    return false;
>> +}
>> +
>>  /*
>>   * Increments nlookup on the inode on success. unref_inode_lolocked() must be
>>   * called eventually to decrement nlookup again. If inodep is non-NULL, the
>> @@ -1116,6 +1145,10 @@ static int lo_do_lookup(fuse_req_t req, fuse_ino_t parent, const char *name,
>>      }
>>      e->ino = inode->fuse_ino;
>>  
>> +    if (lo_should_enable_dax(lo, inode, e)) {
>> +        e->attr_flags |= FUSE_ATTR_DAX;
>> +    }
>> +
> 
> If we were too strict then we should probably add a
> lo_should_enable_dax() call in lo_getattr() as well?

Good catch. FUSE_GETATTR could also reply the per inode DAX flag.

> 
> I guess for now it is ok. Overhead of checking fs_xflag_dax seems
> enough and not many might care about immediate chnage of dax
> status in guest.
> 

OK, we can easily add it if it's needed later.

-- 
Thanks,
Jeffle