Re: [Virtio-fs] [PATCH] virtiofsd: Reset O_DIRECT flag during file open

[piaojun <piaojun@huawei.com>]

On 2019/8/21 2:36, Vivek Goyal wrote:
> On Tue, Aug 20, 2019 at 10:33:29PM +0800, piaojun wrote:
>> Hi Vivek & Aneesh,
>>
>> On 2019/8/20 21:45, Vivek Goyal wrote:
>>> If an application wants to do direct IO and opens a file with O_DIRECT
>>> in guest, that does not necessarily mean that we need to bypass page
>>> cache on host as well. So reset this flag on host.
>>
>> I have a different understanding about this. The I/O mode should be kept
>> same both in Guest and Host. When doing direct io in Guest, the user may
>> want the data go to disk rather than kept in Host page cache which means
>> volatile. So it seems more reasonable that Guest and Host have the same
>> file I/0 semantics.
> 
> I look at it other way. If we open a file O_DIRECT, it should bypass
> page cache on kernel it is running on. It does not guarantee anything
> on underlying storage stack. For example, virtual block device or physical
> disks could still be caching data with O_DIRECT.
> 
> To me same analogy should apply for caching on host. Atleast by default
> O_DIRECT should only mean bypassing page cache in guest. If there is
> an application which requires bypassing page cache on host as well, then
> it should be a server option to control this behavior. But we can do when
> there is a real user.

This strategy also looks good to me if the user could control caches
both in Guest and Host separatly. So the knob you mentioned will be OK.

> 
>>
>>>
>>> If somebody needs to bypass page cache on host as well (and it is safe to
>>> do so), we can add a knob in daemon later to control this behavior.
>>
>> This discussion has some relationship with my RFC PATCH "virtiofsd: do
>> not fall back to buffer io when cache=auto/always", and we could involve
>> the issue mentioned in that email if you do not mind.
>>
> 
> I responded to your patch separately and that seems completely unrelated
> to this patch.

OK, I will reply in my patch.

Jun