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:
>>>  
>>> 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.

> Kevin
>