Re: [PATCH] Use qemu_memalign instead of qemu_malloc

[Anthony Liguori <aliguori@us.ibm.com>]

Kevin Wolf wrote:
> Anthony Liguori schrieb:
>   
>> Kevin Wolf wrote:
>>     
>>> Anthony Liguori schrieb:
>>>  
>>>       
>>>> Kevin Wolf wrote:
>>>>    
>>>>         
>>>>> Anthony Liguori schrieb:
>>>>>  
>>>>> Yes, if it fails, the EINVAL is no surprise. I meant what code path it
>>>>> was using. Obviously we missed something in our patch and I'd like to
>>>>> fix that. Did the error occur on raw images or something like qcow2?
>>>>>         
>>>>>           
>>>> It's a raw image and the calls are being made via
>>>> bdrv_aio_read/bdrv_aio_write.  It doesn't occur with a qcow2 but then
>>>> cache=off doesn't seem to do what it's supposed to with cache=off (I
>>>> believe the underlying backing file is not opened O_DIRECT?).
>>>>     
>>>>         
>>> This is really strange. In raw_aio_read/write there is a check like this:
>>>
>>>     if (unlikely(s->aligned_buf != NULL && ((uintptr_t) buf % 512))) {
>>>         // emulate it using raw_pread/write which uses
>>>         // s->aligned_buf for the request then
>>>     }
>>>   
>>>       
>> Something is goofy then.
>>
>>     
>>> For qcow2 I think O_DIRECT actually is in effect. Otherwise it would
>>> have worked even without our patch, and it didn't. And indeed, looking
>>> at the code, it passes flags to bdrv_file_open when it opens the image
>>> file.
>>>   
>>>       
>> Something's broken then.  Maybe -snapshot doesn't pick up the
>> O_DIRECT'ness?  I'll have to check again.  I was definitely seeing page
>> cache behavior with cache=off.
>>     
>
> Right, qemu seems to drop the flags for the backing file when using
> BDRV_O_SNAPSHOT (bdrv2_open in block.c opens the file). So O_DIRECT
> applies only to new data.
>
> Have you been using -snapshot when you had trouble with the unaligned
> buffer, too? I don't think I have tested this one when I made the patch...
>   

Nope.  I was using a raw image.  Actually, an LVM partition.

Regards,

Anthony Liguori

> Kevin
>