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Re: [PATCH v11 13/13] block: apply COR-filter to block-stream jobs

From: Andrey Shinkevich
Subject: Re: [PATCH v11 13/13] block: apply COR-filter to block-stream jobs
Date: Fri, 16 Oct 2020 18:06:03 +0300
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:68.0) Gecko/20100101 Thunderbird/68.9.0

On 15.10.2020 20:16, Andrey Shinkevich wrote:
On 14.10.2020 19:24, Max Reitz wrote:
On 12.10.20 19:43, Andrey Shinkevich wrote:


  block/stream.c             | 93 +++++++++++++++++++++++++++++-----------------
  tests/qemu-iotests/030     | 51 +++----------------------
  tests/qemu-iotests/030.out |  4 +-
  tests/qemu-iotests/141.out |  2 +-
  tests/qemu-iotests/245     | 19 +++++++---
  5 files changed, 81 insertions(+), 88 deletions(-)

Looks like stream_run() could be a bit streamlined now (the allocation
checking should be unnecessary, unconditionally calling
stream_populate() should be sufficient), but not necessary now.

That is what I had kept in my mind when I tackled this patch. But there is an underwater reef to streamline. Namely, how the block-stream job gets known about a long unallocated tail to exit the loop earlier in the stream_run(). Shall we return the '-EOF' or another error code from the cor_co_preadv_part() to be handled by the stream_run()? Any other suggestions, if any, will be appreciated.

diff --git a/block/stream.c b/block/stream.c
index d3e1812..93564db 100644
--- a/block/stream.c
+++ b/block/stream.c


+    cor_filter_bs = bdrv_cor_filter_append(bs, opts, BDRV_O_RDWR, errp);
+    if (cor_filter_bs == NULL) {
+        goto fail;
+    }
+    if (bdrv_freeze_backing_chain(cor_filter_bs, bs, errp) < 0) {

Is there a reason why we can’t combine this with the
bdrv_free_backing_chain() from bs down to above_base?  I mean, the
effect should be the same, just asking.

The bdrv_freeze_backing_chain(bs, above_base, errp) is called before the bdrv_reopen_set_read_only() to keep the backing chain safe during the context switch. Then we will want to freeze the 'COR -> TOP BS' link as well. Freezing/unfreezing parts is simlier to manage than doing that with the whole chain. If we decide to invoke the bdrv_reopen_set_read_only() after freezing the backing chain together with the COR-filter, we will not be able to get the 'write' permission on the read-only node.

+        bdrv_cor_filter_drop(cor_filter_bs);
+        cor_filter_bs = NULL;
+        goto fail;
+    }
+    s = block_job_create(job_id, &stream_job_driver, NULL, cor_filter_bs,
+                         BLK_PERM_CONSISTENT_READ,
+                         basic_flags | BLK_PERM_WRITE | BLK_PERM_GRAPH_MOD,

Not that I’m an expert on the GRAPH_MOD permission, but why is this
shared here but not below?  Shouldn’t it be the same in both cases?
(Same for taking it as a permission.)

When we invoke the block_job_add_bdrv(&s->common, "active node", bs,..) below (particularly, we need it to block the operations on the top node, bdrv_op_block_all()), we ask for the GRAPH_MOD permission for the top node. To allow that, the parent filter node should share that permission for the underlying node. Otherwise, we get assertion failed in the bdrv_check_update_perm() called from bdrv_replace_node() when we remove the filter.

I will add my comments above to the code.



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