Re: [Qemu-devel] [PATCH v8 7/7] block/backup: use backup-top instead of write notifiers

[Max Reitz <mreitz@redhat.com>]

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On 29.05.19 17:46, Vladimir Sementsov-Ogievskiy wrote:
> Drop write notifiers and use filter node instead. Changes:
> 
> 1. copy-before-writes now handled by filter node, so, drop all
>    is_write_notifier arguments.
> 
> 2. we don't have intersecting requests, so their handling is dropped.
> Instead, synchronization works as follows:
> when backup or backup-top starts copying of some area it firstly
> clears copy-bitmap bits, and nobody touches areas, not marked with
> dirty bits in copy-bitmap, so there is no intersection. Also, backup
> job copy operations are surrounded by bdrv region lock, which is
> actually serializing request, to not interfere with guest writes and
> not read changed data from source (before reading we clear
> corresponding bit in copy-bitmap, so, this area is not more handled by
> backup-top).
> 
> 3. To sync with in-flight requests at job finish we now have drained
> removing of the filter, we don't need rw-lock.
> 
> == RFC part ==
> 
> iotests changed:
> 56: op-blocker doesn't shot now, as we set it on source, but then check
> on filter, when trying to start second backup... Should I workaround it
> somehow?

Hm.  Where does that error message even come from?  The fact that the
target image is in use already (Due to file locks)?

It appears that way indeed.

It seems reasonable to me that you can now run a backup on top of
another backup.  Well, I mean, it is a stupid thing to do, but I don’t
see why the block layer would forbid doing so.

So the test seems superfluous to me.  If we want to keep it (why not),
it should test the opposite, namely that a backup to a different image
(with a different job ID) works.  (It seems simple enough to modify the
job that way, so why not.)

> 129: Hmm, now it is not busy at this moment.. But it's illegal to check
> busy, as job has pause-points and set busy to false in these points.
> Why we assert it in this test?

Nobody knows, it’s probably wrong.  All I know is that 129 is just
broken anyway.

> 141: Obvious, as drv0 is not root node now, but backing of the filter,
> when we try to remove it.

I get a failed assertion in 256.  That is probably because the
bdrv_set_aio_context() calls weren’t as unnecessary as I deemed them to be.

> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>  block/backup.c             | 171 ++++++++++++++-----------------------
>  tests/qemu-iotests/056     |   2 +-
>  tests/qemu-iotests/129     |   1 -
>  tests/qemu-iotests/141.out |   2 +-
>  4 files changed, 68 insertions(+), 108 deletions(-)

For some reason, my gcc starts to complain that backup_loop() may not
initialize error_is_read after this patch.  I don’t know why that is.
Perhaps it inlines backup_do_cow() now?  (So before it just saw that a
pointer to error_is_read was passed to backup_do_cow() and took it as an
opaque function, so it surely would set this value somewhere.  Now it
inlines it and it can’t find whether that will definitely happen, so it
complains.)

I don’t think it is strictly necessary to initialize error_is_read, but,
well, it won’t hurt.

> diff --git a/block/backup.c b/block/backup.c
> index 00f4f8af53..a5b8e04c9c 100644
> --- a/block/backup.c
> +++ b/block/backup.c

[...]

> @@ -60,56 +53,17 @@ typedef struct BackupBlockJob {
>  
>  static const BlockJobDriver backup_job_driver;
>  
> -/* See if in-flight requests overlap and wait for them to complete */
> -static void coroutine_fn wait_for_overlapping_requests(BackupBlockJob *job,
> -                                                       int64_t start,
> -                                                       int64_t end)
> -{
> -    CowRequest *req;
> -    bool retry;
> -
> -    do {
> -        retry = false;
> -        QLIST_FOREACH(req, &job->inflight_reqs, list) {
> -            if (end > req->start_byte && start < req->end_byte) {
> -                qemu_co_queue_wait(&req->wait_queue, NULL);
> -                retry = true;
> -                break;
> -            }
> -        }
> -    } while (retry);
> -}
> -
> -/* Keep track of an in-flight request */
> -static void cow_request_begin(CowRequest *req, BackupBlockJob *job,
> -                              int64_t start, int64_t end)
> -{
> -    req->start_byte = start;
> -    req->end_byte = end;
> -    qemu_co_queue_init(&req->wait_queue);
> -    QLIST_INSERT_HEAD(&job->inflight_reqs, req, list);
> -}
> -
> -/* Forget about a completed request */
> -static void cow_request_end(CowRequest *req)
> -{
> -    QLIST_REMOVE(req, list);
> -    qemu_co_queue_restart_all(&req->wait_queue);
> -}
> -
>  /* Copy range to target with a bounce buffer and return the bytes copied. If
>   * error occurred, return a negative error number */
>  static int coroutine_fn backup_cow_with_bounce_buffer(BackupBlockJob *job,
>                                                        int64_t start,
>                                                        int64_t end,
> -                                                      bool is_write_notifier,
>                                                        bool *error_is_read,
>                                                        void **bounce_buffer)

Future feature: Somehow get this functionality done with backup-top, I
suppose.  (This is effectively just backup_top_cbw() with some bells and
whistles, isn’t it?)

>  {
>      int ret;
>      BlockBackend *blk = job->common.blk;
>      int nbytes;
> -    int read_flags = is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0;
>      int write_flags = job->serialize_target_writes ? BDRV_REQ_SERIALISING : 0;
>  
>      assert(QEMU_IS_ALIGNED(start, job->cluster_size));

[...]

> @@ -154,15 +108,12 @@ fail:
>  /* Copy range to target and return the bytes copied. If error occurred, return a
>   * negative error number. */
>  static int coroutine_fn backup_cow_with_offload(BackupBlockJob *job,
> -                                                int64_t start,
> -                                                int64_t end,
> -                                                bool is_write_notifier)
> +                                                int64_t start, int64_t end)

And I suppose this is something backup-top maybe should support, too.

>  {
>      int ret;
>      int nr_clusters;
>      BlockBackend *blk = job->common.blk;
>      int nbytes;
> -    int read_flags = is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0;
>      int write_flags = job->serialize_target_writes ? BDRV_REQ_SERIALISING : 0;
>  
>      assert(QEMU_IS_ALIGNED(job->copy_range_size, job->cluster_size));

[...]

> @@ -391,28 +333,41 @@ static int coroutine_fn backup_loop(BackupBlockJob *job)
>      int64_t offset;
>      HBitmapIter hbi;
>      BlockDriverState *bs = blk_bs(job->common.blk);
> +    void *lock;
>  
>      hbitmap_iter_init(&hbi, job->copy_bitmap, 0);
>      while ((offset = hbitmap_iter_next(&hbi)) != -1) {
> +        lock = bdrv_co_try_lock(backing_bs(blk_bs(job->common.blk)), offset,
> +                                job->cluster_size);
> +        /*
> +         * Dirty bit is set, which means that there are no in-flight
> +         * write requests on this area. We must succeed.
> +         */
> +        assert(lock);
> +

Hm.  It makes me uneasy but I suppose you’re right.

>          if (job->sync_mode == MIRROR_SYNC_MODE_TOP &&
>              bdrv_is_unallocated_range(bs, offset, job->cluster_size))

This can yield, right?  If it does, the bitmap is still set.  backup-top
will see this, unset the bitmap and try to start its CBW operation.
That is halted by the lock just taken, but the progress will still be
published after completion, so the job can go beyond 100 %, I think.

Even if it doesn’t, copying the data twice is weird.  It may even get
weirder if one of both requests fails.

Can we lock the backup-top node instead?  I don’t know whether locking
would always succeed there, though...

Max

>          {
>              hbitmap_reset(job->copy_bitmap, offset, job->cluster_size);
> +            bdrv_co_unlock(lock);
>              continue;
>          }
>  
>          do {
>              if (yield_and_check(job)) {
> +                bdrv_co_unlock(lock);
>                  return 0;
>              }
> -            ret = backup_do_cow(job, offset,
> -                                job->cluster_size, &error_is_read, false);
> +            ret = backup_do_cow(job, offset, job->cluster_size, &error_is_read);
>              if (ret < 0 && backup_error_action(job, error_is_read, -ret) ==
>                             BLOCK_ERROR_ACTION_REPORT)
>              {
> +                bdrv_co_unlock(lock);
>                  return ret;
>              }
>          } while (ret < 0);
> +
> +        bdrv_co_unlock(lock);
>      }
>  
>      return 0;


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