Re: [Qemu-devel] [PATCH v4 02/11] block: Filtered children access functions

[Max Reitz <mreitz@redhat.com>]

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On 31.05.19 18:26, Max Reitz wrote:
> On 16.04.19 12:02, Vladimir Sementsov-Ogievskiy wrote:
>> 10.04.2019 23:20, Max Reitz wrote:
>>> What bs->file and bs->backing mean depends on the node.  For filter
>>> nodes, both signify a node that will eventually receive all R/W
>>> accesses.  For format nodes, bs->file contains metadata and data, and
>>> bs->backing will not receive writes -- instead, writes are COWed to
>>> bs->file.  Usually.
>>>
>>> In any case, it is not trivial to guess what a child means exactly with
>>> our currently limited form of expression.  It is better to introduce
>>> some functions that actually guarantee a meaning:
>>>
>>> - bdrv_filtered_cow_child() will return the child that receives requests
>>>    filtered through COW.  That is, reads may or may not be forwarded
>>>    (depending on the overlay's allocation status), but writes never go to
>>>    this child.
>>>
>>> - bdrv_filtered_rw_child() will return the child that receives requests
>>>    filtered through some very plain process.  Reads and writes issued to
>>>    the parent will go to the child as well (although timing, etc. may be
>>>    modified).
>>>
>>> - All drivers but quorum (but quorum is pretty opaque to the general
>>>    block layer anyway) always only have one of these children: All read
>>>    requests must be served from the filtered_rw_child (if it exists), so
>>>    if there was a filtered_cow_child in addition, it would not receive
>>>    any requests at all.
>>>    (The closest here is mirror, where all requests are passed on to the
>>>    source, but with write-blocking, write requests are "COWed" to the
>>>    target.  But that just means that the target is a special child that
>>>    cannot be introspected by the generic block layer functions, and that
>>>    source is a filtered_rw_child.)
>>>    Therefore, we can also add bdrv_filtered_child() which returns that
>>>    one child (or NULL, if there is no filtered child).
>>>
>>> Also, many places in the current block layer should be skipping filters
>>> (all filters or just the ones added implicitly, it depends) when going
>>> through a block node chain.  They do not do that currently, but this
>>> patch makes them.
>>>
>>> One example for this is qemu-img map, which should skip filters and only
>>> look at the COW elements in the graph.  The change to iotest 204's
>>> reference output shows how using blkdebug on top of a COW node used to
>>> make qemu-img map disregard the rest of the backing chain, but with this
>>> patch, the allocation in the base image is reported correctly.
>>>
>>> Furthermore, a note should be made that sometimes we do want to access
>>> bs->backing directly.  This is whenever the operation in question is not
>>> about accessing the COW child, but the "backing" child, be it COW or
>>> not.  This is the case in functions such as bdrv_open_backing_file() or
>>> whenever we have to deal with the special behavior of @backing as a
>>> blockdev option, which is that it does not default to null like all
>>> other child references do.
>>>
>>> Finally, the query functions (query-block and query-named-block-nodes)
>>> are modified to return any filtered child under "backing", not just
>>> bs->backing or COW children.  This is so that filters do not interrupt
>>> the reported backing chain.  This changes the output of iotest 184, as
>>> the throttled node now appears as a backing child.
>>>
>>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>>> ---
>>>   qapi/block-core.json           |   4 +
>>>   include/block/block.h          |   1 +
>>>   include/block/block_int.h      |  40 +++++--
>>>   block.c                        | 210 +++++++++++++++++++++++++++------
>>>   block/backup.c                 |   8 +-
>>>   block/block-backend.c          |  16 ++-
>>>   block/commit.c                 |  33 +++---
>>>   block/io.c                     |  45 ++++---
>>>   block/mirror.c                 |  21 ++--
>>>   block/qapi.c                   |  30 +++--
>>>   block/stream.c                 |  13 +-
>>>   blockdev.c                     |  88 +++++++++++---
>>>   migration/block-dirty-bitmap.c |   4 +-
>>>   nbd/server.c                   |   6 +-
>>>   qemu-img.c                     |  29 ++---
>>>   tests/qemu-iotests/184.out     |   7 +-
>>>   tests/qemu-iotests/204.out     |   1 +
>>>   17 files changed, 411 insertions(+), 145 deletions(-)
>>
>> really huge... didn't you consider conversion file-by-file?
>>
>> [..]
>>
>>> diff --git a/block.c b/block.c
>>> index 16615bc876..e8f6febda0 100644
>>> --- a/block.c
>>> +++ b/block.c
>>
>> [..]
>>
>>>   
>>> @@ -3467,14 +3469,17 @@ static int bdrv_reopen_parse_backing(BDRVReopenState *reopen_state,
>>>       /*
>>>        * Find the "actual" backing file by skipping all links that point
>>>        * to an implicit node, if any (e.g. a commit filter node).
>>> +     * We cannot use any of the bdrv_skip_*() functions here because
>>> +     * those return the first explicit node, while we are looking for
>>> +     * its overlay here.
>>>        */
>>>       overlay_bs = bs;
>>> -    while (backing_bs(overlay_bs) && backing_bs(overlay_bs)->implicit) {
>>> -        overlay_bs = backing_bs(overlay_bs);
>>> +    while (overlay_bs->backing && bdrv_filtered_bs(overlay_bs)->implicit) {
>>
>> So, you don't want to skip implicit filters with 'file' child? Then, why not to use
>> child_bs(overlay_bs->backing), like in following if condition?
> 
> On second thought, I actually think this version is wrong in the other way.
> 
> There needs to be a bs with bs->backing != NULL and !bs->implicit
> somewhere in the chain.

(Actually, no, the bs->backing node is @bs)

> We try to find that node.  It doesn’t matter
> what’s on top of it, though,  If there are implicit node (which we try
> to skip here), the user isn’t aware of them.  Consequentially, it
> doesn’t matter whether these implicit nodes use bs->backing or bs->file,
> we just need to skip them.
> 
> What is wrong is the “while (overlay_bs->backing ...)”.  That needs to
> be “while (bdrv_filtered_bs(overlay_bs) ...)”.

I just saw my reply where I noticed this before...  So nothing too new then.

Max


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