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

[Yehuda Sadeh Weinraub]

On Thu, Oct 7, 2010 at 11:38 AM, Anthony Liguori <address@hidden> wrote:
> On 10/07/2010 01:08 PM, Yehuda Sadeh Weinraub wrote:
>>
>> On Thu, Oct 7, 2010 at 7:12 AM, Anthony Liguori<address@hidden>
>>  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
>
(resending to list, sorry)

The fd is hidden deep under in librados. We get callback notifications
for events completion.

Thanks,
Yehuda