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

[Daniel P. Berrange]

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.

Regards,
Daniel
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