Re: [Virtio-fs] [PATCH][RFC] viriofsd: do not fall back to buffer io when cache=auto

[piaojun <piaojun@huawei.com>]

On 2019/8/19 11:15, Eryu Guan wrote:
> On Wed, Aug 14, 2019 at 05:46:42PM +0800, piaojun wrote:
>> Direct io flags will be tured off even if cache=auto, which seems a
>> little bit strange. It's better to keep the open flags set by user. If
>> I missed the discussion about this issue, please let me know, thanks.
>>
>> Signed-off-by: Jun Piao <piaojun@huawei.com>
>> ---
>>  contrib/virtiofsd/passthrough_ll.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/contrib/virtiofsd/passthrough_ll.c b/contrib/virtiofsd/passthrough_ll.c
>> index 321bbb2..a080f0d 100644
>> --- a/contrib/virtiofsd/passthrough_ll.c
>> +++ b/contrib/virtiofsd/passthrough_ll.c
>> @@ -1687,6 +1687,8 @@ static void lo_create(fuse_req_t req, fuse_ino_t parent, const char *name,
>>  		fi->direct_io = 1;
>>  	else if (lo->cache == CACHE_ALWAYS)
>>  		fi->keep_cache = 1;
>> +	else
>> +		fi->direct_io = !!(fi->flags & O_DIRECT);
>>
>>  out:
>>  	lo_inode_put(lo, &parent_inode);
>> @@ -1899,6 +1901,8 @@ static void lo_open(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi)
>>  		fi->direct_io = 1;
>>  	else if (lo->cache == CACHE_ALWAYS)
>>  		fi->keep_cache = 1;
>> +	else
>> +		fi->direct_io = !!(fi->flags & O_DIRECT);
>>  	fuse_reply_open(req, fi);
> 
> IMHO, this is necessary, it seems that in the case cache=auto/always,
> virtiofsd already follows what guest requests, i.e.  virtiofsd only does
> direct I/O when guest opens file with O_DIRECT.

>From my testing, virtiofsd won't do direct io even if guest opens file
with O_DIRECT when cache=auto/alway. It's weird to convert direct io to
buffer io at host side.

> 
> If I understand the code correctly, fi->direct_io = 1 is used to force
> guest to use direct I/O even if guest opens file without O_DIRECT.

But in fact, direct I/O will be forced to buffer I/O even if with
O_DIRECT. I will look into this problem by further testing, and very
glad to share my test result.

Jun