Re: [Qemu-devel] [PATCH] 9pfs: don't ignore O_DIRECT flag in the 9pfs server

[jiangyiwen <jiangyiwen@huawei.com>]

On 2017/11/20 18:13, Greg Kurz wrote:
> On Mon, 20 Nov 2017 13:48:59 +0800
> jiangyiwen <jiangyiwen@huawei.com> wrote:
> 
>> Now v9fs in linux has already supported O_DIRECT(v9fs_direct_IO),
>> when guest user open file with O_DIRECT flag and return success,
>> so user hopes data doesn't pass through page cache, but 9pfs in
>> qemu ignore direct disk access and use host page cache, it is not
>> match to DIRECT_IO semantic, so we should not ignore O_DIRECT in
>> 9pfs unless v9fs in linux don't support DIRECT_IO.
>>
>> And if server fs don't support O_DIRECT, user will receive -EINVAL
>> and know the filesystem don't support O_DIRECT.
>>
>> So in order to ensure semantic consistency, don't ignore direct
>> disk access in 9pfs.
>>
> 
> There are good reasons for us to ignore O_DIRECT. AFAIK, O_DIRECT requires
> the application to take care of the alignment of the buffers with either the
> logical block size of the filesystem (linux <= 2.4) or the logical block size
> of the underlying storage... I don't really see how you can achieve that since
> the linux 9p client only cares to break up requests into msize-chunks, and
> ignores alignment. ie, you're likely to not ensure semantic consistency on the
> host side anyway and linux will fallback to cached I/O.
> 
> Also, this change would silently break existing users of O_DIRECT, which isn't
> acceptable... it would require some fsdev property to enable this new behavior.
> 

Thank you very much, I understand what your mean, v9fs in linux doesn't take care
of the alignment, only split as the msize. Then there is another problem,
should v9fs support DIRECT_IO? or delete DIRECT_IO ops in v9fs?

Thanks,
Yiwen.
>> Signed-off-by: Yiwen Jiang <jiangyiwen@huawei.com>
>> ---
>>  hw/9pfs/9p.c | 4 ----
>>  1 file changed, 4 deletions(-)
>>
>> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
>> index 52d4663..5ea01c4 100644
>> --- a/hw/9pfs/9p.c
>> +++ b/hw/9pfs/9p.c
>> @@ -155,10 +155,6 @@ static int get_dotl_openflags(V9fsState *s, int oflags)
>>       */
>>      flags = dotl_to_open_flags(oflags);
>>      flags &= ~(O_NOCTTY | O_ASYNC | O_CREAT);
>> -    /*
>> -     * Ignore direct disk access hint until the server supports it.
>> -     */
>> -    flags &= ~O_DIRECT;
>>      return flags;
>>  }
>>
> 
> 
> .
>