Re: [Virtio-fs] [PATCH] virtiofsd: Fix data corruption with O_APPEND wirte in writeback mode

[Vivek Goyal]

On Thu, Oct 31, 2019 at 09:39:23AM +0000, address@hidden wrote:
[..]
> > > > Hi Misono,
> > > >
> > > > Have you tried running pjdfstests. Looks like with the patch applied
> > > > I see following tests failing which were passing without the patch.
> > > > Can you please have a look. I ran daemon with options "-o
> > > cache=always -o writeback"
> > 
> > I see these errors in both with and without this patch but not always.
> > Do you always see the failure?
> > 
> > I use:
> >   Kernel: Fuse's for-next branch
> >   Qemu: virtio-fs-dev branch
> >   backend: XFS (relatime)
> > 
> > These tests fail because a/c/m time is not updated as expected.
> > So it seems this is related to the failure of xfstest generic/003.
> > I will look into the problem more.
> > 
> 
> Hi,
> 
> So I fugured out the problem. 
> The failure occurs when guest clock is earlier than host clock in writeback 
> mode.
> 
> The failure case checks:
>  1. get current c/time of file
>  2. sleep 1 second
>  3. modify file's metadata/data
>  4. check if c/mtime is updated
> 
> And the tests fail because:
>  1. when file is opened, guest inode's time is initialized by host's inode 
> information
>  2. In writeback mode, guest clock is used for c/mtime update
>  3. if guest clock is earlier than host clock, c/mtime could be updated
>     earlier than current inode time. 
> 
> I can reproduce the problem reliably by deliberately setting guest clock 
> earlier
> than host clock. I'm not sure if virtiofs is to be blmaed in this case, but 
> should
> we not update c/mtime in guest if it is earlier then current c/mtime?
> 
> Anyway, I believe this O_APPEND fix patch is irrelevant to the problem,
> could you please check it again?

Hi Misono,

Thanks for looking into it. I agree that a/c/m time updates with -o writeback
is probably a different issue and not related to your patch. So I am fine
with your patch.

Thanks
Vivek