Re: [PATCH] Use qemu_memalign instead of qemu_malloc

[Kevin Wolf <kwolf@suse.de>]

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...

Kevin