Re: [PATCH V2] block/rbd: implement bdrv_co_block_status

[Ilya Dryomov]

On Tue, Aug 10, 2021 at 3:41 PM Peter Lieven <pl@kamp.de> wrote:
>
> the qemu rbd driver currently lacks support for bdrv_co_block_status.
> This results mainly in incorrect progress during block operations (e.g.
> qemu-img convert with an rbd image as source).
>
> This patch utilizes the rbd_diff_iterate2 call from librbd to detect
> allocated and unallocated (all zero areas).
>
> To avoid querying the ceph OSDs for the answer this is only done if
> the image has the fast-diff features which depends on the object-map

Hi Peter,

Nit: "has the fast-diff feature which depends on the object-map and
exclusive-lock features"

> and exclusive-lock. In this case it is guaranteed that the information
> is present in memory in the librbd client and thus very fast.
>
> If fast-diff is not available all areas are reported to be allocated
> which is the current behaviour if bdrv_co_block_status is not implemented.
>
> Signed-off-by: Peter Lieven <pl@kamp.de>
> ---
> V1->V2:
> - add commit comment [Stefano]
> - use failed_post_open [Stefano]
> - remove redundant assert [Stefano]
> - add macro+comment for the magic -9000 value [Stefano]
> - always set *file if its non NULL [Stefano]
>
>  block/rbd.c | 125 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 125 insertions(+)
>
> diff --git a/block/rbd.c b/block/rbd.c
> index dcf82b15b8..8692e76f40 100644
> --- a/block/rbd.c
> +++ b/block/rbd.c
> @@ -88,6 +88,7 @@ typedef struct BDRVRBDState {
>      char *namespace;
>      uint64_t image_size;
>      uint64_t object_size;
> +    uint64_t features;
>  } BDRVRBDState;
>
>  typedef struct RBDTask {
> @@ -983,6 +984,13 @@ static int qemu_rbd_open(BlockDriverState *bs, QDict 
> *options, int flags,
>      s->image_size = info.size;
>      s->object_size = info.obj_size;
>
> +    r = rbd_get_features(s->image, &s->features);
> +    if (r < 0) {
> +        error_setg_errno(errp, -r, "error getting image features from %s",
> +                         s->image_name);
> +        goto failed_post_open;
> +    }

The object-map and fast-diff features can be enabled/disabled while the
image is open so this should probably go to qemu_rbd_co_block_status().

> +
>      /* If we are using an rbd snapshot, we must be r/o, otherwise
>       * leave as-is */
>      if (s->snap != NULL) {
> @@ -1259,6 +1267,122 @@ static ImageInfoSpecific 
> *qemu_rbd_get_specific_info(BlockDriverState *bs,
>      return spec_info;
>  }
>
> +typedef struct rbd_diff_req {
> +    uint64_t offs;
> +    uint64_t bytes;
> +    int exists;
> +} rbd_diff_req;
> +
> +/*
> + * rbd_diff_iterate2 allows to interrupt the exection by returning a negative
> + * value in the callback routine. Choose a value that does not conflict with
> + * an existing exitcode and return it if we want to prematurely stop the
> + * execution because we detected a change in the allocation status.
> + */
> +#define QEMU_RBD_EXIT_DIFF_ITERATE2 -9000
> +
> +static int qemu_rbd_co_block_status_cb(uint64_t offs, size_t len,
> +                                       int exists, void *opaque)
> +{
> +    struct rbd_diff_req *req = opaque;
> +
> +    assert(req->offs + req->bytes <= offs);
> +
> +    if (req->exists && offs > req->offs + req->bytes) {
> +        /*
> +         * we started in an allocated area and jumped over an unallocated 
> area,
> +         * req->bytes contains the length of the allocated area before the
> +         * unallocated area. stop further processing.
> +         */
> +        return QEMU_RBD_EXIT_DIFF_ITERATE2;
> +    }
> +    if (req->exists && !exists) {
> +        /*
> +         * we started in an allocated area and reached a hole. req->bytes
> +         * contains the length of the allocated area before the hole.
> +         * stop further processing.
> +         */
> +        return QEMU_RBD_EXIT_DIFF_ITERATE2;
> +    }
> +    if (!req->exists && exists && offs > req->offs) {
> +        /*
> +         * we started in an unallocated area and hit the first allocated
> +         * block. req->bytes must be set to the length of the unallocated 
> area
> +         * before the allocated area. stop further processing.
> +         */
> +        req->bytes = offs - req->offs;
> +        return QEMU_RBD_EXIT_DIFF_ITERATE2;
> +    }
> +
> +    /*
> +     * assert that we catched all cases above and allocation state has not

catched -> caught

> +     * changed during callbacks.
> +     */
> +    assert(exists == req->exists || !req->bytes);
> +    req->exists = exists;
> +
> +    /*
> +     * assert that we either return an unallocated block or have got 
> callbacks
> +     * for all allocated blocks present.
> +     */
> +    assert(!req->exists || offs == req->offs + req->bytes);
> +    req->bytes = offs + len - req->offs;
> +
> +    return 0;
> +}
> +
> +static int coroutine_fn qemu_rbd_co_block_status(BlockDriverState *bs,
> +                                                 bool want_zero, int64_t 
> offset,
> +                                                 int64_t bytes, int64_t 
> *pnum,
> +                                                 int64_t *map,
> +                                                 BlockDriverState **file)
> +{
> +    BDRVRBDState *s = bs->opaque;
> +    int ret, r;
> +    struct rbd_diff_req req = { .offs = offset };
> +
> +    assert(offset + bytes <= s->image_size);
> +
> +    /* default to all sectors allocated */
> +    ret = BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID;

I'm a little confused by the meaning of these flags (but I haven't
looked at the other drivers yet).  Looks like this patch always sets
BDRV_BLOCK_OFFSET_VALID (makes sense since the "host" offset is always
known for rbd) and returns either BDRV_BLOCK_DATA or BDRV_BLOCK_ZERO.

DATA ZERO OFFSET_VALID
 t    t        t       sectors read as zero, returned file is zero at offset
 t    f        t       sectors read as valid from file at offset
 f    t        t       sectors preallocated, read as zero, returned file not
                       necessarily zero at offset
 f    f        t       sectors preallocated but read from backing_hd,
                       returned file contains garbage at offset

What about the first case (BDRV_BLOCK_DATA | BDRV_BLOCK_ZERO)?
What is the practical difference to just BDRV_BLOCK_ZERO?

> +    if (map) {
> +        *map = offset;
> +    }
> +    if (file) {
> +        *file = bs;
> +    }

A comment in block_int.h says that map and file are guaranteed to be
non-NULL so these tests seem redundant?

> +    *pnum = bytes;
> +
> +    /* RBD image does not support fast-diff */
> +    if (!(s->features & RBD_FEATURE_FAST_DIFF)) {
> +        goto out;
> +    }

Need to make sure that fast-diff is valid here: call rbd_get_flags()
on the image and test for !RBD_FLAG_FAST_DIFF_INVALID.

> +
> +    r = rbd_diff_iterate2(s->image, NULL, offset, bytes, true, true,
> +                          qemu_rbd_co_block_status_cb, &req);
> +    if (r < 0 && r != QEMU_RBD_EXIT_DIFF_ITERATE2) {
> +        goto out;
> +    }
> +    assert(req.bytes <= bytes);
> +    if (!req.exists) {
> +        if (r == 0 && !req.bytes) {
> +            /*
> +             * rbd_diff_iterate2 does not invoke callbacks for unallocated 
> areas
> +             * except for the case where an overlay has a hole where the 
> parent
> +             * has not. This here catches the case where no callback was
> +             * invoked at all.
> +             */
> +            req.bytes = bytes;
> +        }
> +        ret &= ~BDRV_BLOCK_DATA;
> +        ret |= BDRV_BLOCK_ZERO;

Is this equivalent to ret = BDRV_BLOCK_ZERO | BDRV_BLOCK_OFFSET_VALID?
If so, that would be clearer IMO.

Thanks,

                Ilya