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

[Nikolay Shirokovskiy]

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.

Nikolay