[libvirt] [PATCH v2] iohelper: fix reading with O_DIRECT

[Daniel P. Berrange]

On Wed, Sep 20, 2017 at 04:45:35PM +0300, Nikolay Shirokovskiy wrote:
> 
> 
> On 20.09.2017 16:41, Daniel P. Berrange wrote:
> > On Wed, Sep 20, 2017 at 04:35:58PM +0300, Nikolay Shirokovskiy wrote:
> >>
> >>
> >> On 20.09.2017 16:30, Daniel P. Berrange wrote:
> >>> On Wed, Sep 20, 2017 at 02:58:55PM +0300, Nikolay Shirokovskiy wrote:
> >>>> saferead is not suitable for direct reads. If file size is not multiple
> >>>> of align size then we get EINVAL on the read(2) that is supposed to
> >>>> return 0 because read buffer will not be aligned at this point.
> >>>>
> >>>> Let's not read again after partial read and check that we read
> >>>> everything by comparing the number of total bytes read against file size.
> >>>
> >>> What scenario did you actually hit this problem in ?    IIUC, we should
> >>> only be using O_DIRECT against block devices or plain files, and in both
> >>> those cases we should never see short-read unless we hit EOF.
> >>
> >> Yes. But saferead is generic function and rereads file after short read.
> >> Here we got EINVAL because of misalignement.
> > 
> > I understand that, but how have you actually hit this in the real world.
> > AFAICT, the short-read and subsequent problem with misalignemt should be
> > impossible to hit, because any files we use O_DIRECT on should not ever
> > return a shortread.
> 
> virsh restore --bypass-cache just fails (at least on my kernel). The problem
> is that if the state file size is not multiple of buffer then last read
> will definetly be short read.

Ah ok I see what's happening - its a short read in the sense that we have
hit end-of-file, not a short read in the middle of the file.

Regards,
Daniel
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