Re: [Qemu-devel] [PATCH] ceph/rbd block driver for qemu-kvm (v4)

[Anthony Liguori <anthony@codemonkey.ws>]

On 10/07/2010 01:08 PM, Yehuda Sadeh Weinraub wrote:
> On Thu, Oct 7, 2010 at 7:12 AM, Anthony Liguori<anthony@codemonkey.ws>  wrote:
>    
>> On 08/03/2010 03:14 PM, Christian Brunner wrote:
>>      
>>> +#include "qemu-common.h"
>>> +#include "qemu-error.h"
>>> +#include<sys/types.h>
>>> +#include<stdbool.h>
>>> +
>>> +#include<qemu-common.h>
>>>
>>>        
>> This looks to be unnecessary.  Generally, system includes shouldn't be
>> required so all of these should go away except rado/librados.h
>>      
> Removed.
>
>    
>>      
>>> +
>>> +#include "rbd_types.h"
>>> +#include "module.h"
>>> +#include "block_int.h"
>>> +
>>> +#include<stdio.h>
>>> +#include<stdlib.h>
>>> +#include<rados/librados.h>
>>> +
>>> +#include<signal.h>
>>> +
>>> +
>>> +int eventfd(unsigned int initval, int flags);
>>>
>>>        
>> This is not quite right.  Depending on eventfd is curious but in the very
>> least, you need to detect the presence of eventfd in configure and provide a
>> wrapper that redefines it as necessary.
>>      
> Can fix that, though please see my later remarks.
>    
>>> +static int create_tmap_op(uint8_t op, const char *name, char **tmap_desc)
>>> +{
>>> +    uint32_t len = strlen(name);
>>> +    /* total_len = encoding op + name + empty buffer */
>>> +    uint32_t total_len = 1 + (sizeof(uint32_t) + len) + sizeof(uint32_t);
>>> +    char *desc = NULL;
>>>
>>>        
>> char is the wrong type to use here as it may be signed or unsigned.  That
>> can have weird effects with binary data when you're directly manipulating
>> it.
>>      
> Well, I can change it to uint8_t, so that it matches the op type, but
> that'll require adding some other castings. In any case, you usually
> get such a weird behavior when you cast to types of different sizes
> and have the sign bit padded which is not the case in here.
>
>    
>>      
>>> +
>>> +    desc = qemu_malloc(total_len);
>>> +
>>> +    *tmap_desc = desc;
>>> +
>>> +    *desc = op;
>>> +    desc++;
>>> +    memcpy(desc,&len, sizeof(len));
>>> +    desc += sizeof(len);
>>> +    memcpy(desc, name, len);
>>> +    desc += len;
>>> +    len = 0;
>>> +    memcpy(desc,&len, sizeof(len));
>>> +    desc += sizeof(len);
>>>
>>>        
>> Shouldn't endianness be a concern?
>>      
> Right. Fixed that.
>
>    
>>      
>>> +
>>> +    return desc - *tmap_desc;
>>> +}
>>> +
>>> +static void free_tmap_op(char *tmap_desc)
>>> +{
>>> +    qemu_free(tmap_desc);
>>> +}
>>> +
>>> +static int rbd_register_image(rados_pool_t pool, const char *name)
>>> +{
>>> +    char *tmap_desc;
>>> +    const char *dir = RBD_DIRECTORY;
>>> +    int ret;
>>> +
>>> +    ret = create_tmap_op(CEPH_OSD_TMAP_SET, name,&tmap_desc);
>>> +    if (ret<    0) {
>>> +        return ret;
>>> +    }
>>> +
>>> +    ret = rados_tmap_update(pool, dir, tmap_desc, ret);
>>> +    free_tmap_op(tmap_desc);
>>> +
>>> +    return ret;
>>> +}
>>>
>>>        
>> This ops are all synchronous?  IOW, rados_tmap_update() call blocks until
>> the operation is completed?
>>      
> Yeah. And this is only called from the rbd_create() callback.
>
>    
>>> +            header_snap += strlen(header_snap) + 1;
>>> +            if (header_snap>    end)
>>> +                error_report("bad header, snapshot list broken");
>>>
>>>        
>> Missing curly braces here.
>>      
> Fixed.
>
>    
>>> +    if (strncmp(hbuf + 68, RBD_HEADER_VERSION, 8)) {
>>> +        error_report("Unknown image version %s", hbuf + 68);
>>> +        r = -EMEDIUMTYPE;
>>> +        goto failed;
>>> +    }
>>> +
>>> +    RbdHeader1 *header;
>>>
>>>
>>>        
>> Don't mix variable definitions with code.
>>      
> Fixed.
>
>    
>>> +    s->efd = eventfd(0, 0);
>>> +    if (s->efd<    0) {
>>> +        error_report("error opening eventfd");
>>> +        goto failed;
>>> +    }
>>> +    fcntl(s->efd, F_SETFL, O_NONBLOCK);
>>> +    qemu_aio_set_fd_handler(s->efd, rbd_aio_completion_cb, NULL,
>>> +        rbd_aio_flush_cb, NULL, s);
>>>
>>>        
>> It looks like you just use the eventfd to signal aio completion callbacks.
>>   A better way to do this would be to schedule a bottom half.  eventfds are
>> Linux specific and specific to recent kernels.
>>      
> Digging back why we introduced the eventfd, it was due to some issues
> seen with do_savevm() hangs on qemu_aio_flush(). The reason seemed
> that we had no fd associated with the block device, which seemed to
> not work well with the qemu aio model. If that assumption is wrong,
> we'd be happy to change it. In any case, there are other more portable
> ways to generate fds, so if it's needed we can do that.
>    

There's no fd at all?   How do you get notifications about an 
asynchronous event completion?

Regards,

Anthony Liguori