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

[Max Reitz <mreitz@redhat.com>]

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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?

Frankly, no, I just didn’t consider it.

Hm.  I don’t know, 30-patch series always look so frightening.

>> 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?

I think it was an artifact of writing the patch.  I started with
bdrv_filtered_bs() and then realized this depends on ->backing,
actually.  There was no functional difference so I left it as it was.

But you’re right, it is more clear to use child_bs(overlay_bs->backing)
isntead.

> Could we instead make backing-based filters equal to file-based, to make it possible
> to use file-based filters in backing-chain related scenarios (like upcoming copy-on-read
> filter for stream)? So, to expand backing-chain concept to include filters with file child?

If I understand you correctly, that’s basically the purpose of this
series and especially this patch here.  As far as it is possible and
reasonable, I want filters that use bs->backing and bs->file behave the
same.

However, there are cases where this is not possible and
bdrv_reopen_parse_backing() is one such case.  bs->backing and bs->file
correspond to QAPI names, namely 'backing' and 'file'.  If that
distinction was already visible to the user, we cannot change it now.

We definitely cannot make file-based filters use bs->backing now because
you can create them over QAPI and they use 'file' as their child name.
Can we make backing-based filters use bs->file?  Seems more likely,
because all of them are implicit nodes, so the user usually doesn’t see
them.  But usually isn’t always; they do become user-visible once the
user specifies a node-name for mirror or commit.

I found it more reasonable to introduce new functions that explicitly
express what kind of child they expect and then apply them everywhere as
I saw fit, instead of making the mirror/commit filter drivers use
bs->file and hope it works; not least because I’d still have to go
through the whole block layer and check every instance of bs->backing to
see whether it really needs bs->backing or whether it should use either
of bs->backing or bs->file.

>> +        overlay_bs = bdrv_filtered_bs(overlay_bs);
>>       }
>>   
>>       /* If we want to replace the backing file we need some extra checks */
>> -    if (new_backing_bs != backing_bs(overlay_bs)) {
>> +    if (new_backing_bs != child_bs(overlay_bs->backing)) { >           /* Check for implicit nodes between bs and its backing file */
>>           if (bs != overlay_bs) {
>>               error_setg(errp, "Cannot change backing link if '%s' has "
> 
> [..]
> 
>> @@ -4203,8 +4208,8 @@ int bdrv_change_backing_file(BlockDriverState *bs,
>>   BlockDriverState *bdrv_find_overlay(BlockDriverState *active,
>>                                       BlockDriverState *bs)
>>   {
>> -    while (active && bs != backing_bs(active)) {
>> -        active = backing_bs(active);
>> +    while (active && bs != bdrv_filtered_bs(active)) {
> 
> hmm and here you actually support backing-chain with file-child-based filters in it..

Yes, because this is not about the QAPI 'backing' link.  This function
should continue to work even if there are filters in the backing chain.

>> +        active = bdrv_filtered_bs(active);
>>       }
>>   
>>       return active;
>> @@ -4226,11 +4231,11 @@ bool bdrv_is_backing_chain_frozen(BlockDriverState *bs, BlockDriverState *base,
>>   {
>>       BlockDriverState *i;
>>   
>> -    for (i = bs; i != base; i = backing_bs(i)) {
>> +    for (i = bs; i != base; i = child_bs(i->backing)) {
> 
> and here don't..

Yes, because this function is about the QAPI 'backing' link.

>>           if (i->backing && i->backing->frozen) {
>>               error_setg(errp, "Cannot change '%s' link from '%s' to '%s'",
>>                          i->backing->name, i->node_name,
>> -                       backing_bs(i)->node_name);
>> +                       i->backing->bs->node_name);
>>               return true;
>>           }
>>       }
> 
> [..]
> 
>> +static BlockDriverState *bdrv_skip_filters(BlockDriverState *bs,
>> +                                           bool stop_on_explicit_filter)
>> +{
>> +    BdrvChild *filtered;
>> +
>> +    if (!bs) {
>> +        return NULL;
>> +    }
>> +
>> +    while (!(stop_on_explicit_filter && !bs->implicit)) {
> 
> you may save some characters and extra operators by
> 
> bool skip_explicit
> ...
> while (skip_explicit || bs->implicit) {

But is it really simpler?

>> +        filtered = bdrv_filtered_rw_child(bs);
>> +        if (!filtered) {
>> +            break;
>> +        }
>> +        bs = filtered->bs;
>> +    }
>> +    /*
>> +     * Note that this treats nodes with bs->drv == NULL as not being
>> +     * R/W filters (bs->drv == NULL should be replaced by something
>> +     * else anyway).
>> +     * The advantage of this behavior is that this function will thus
>> +     * always return a non-NULL value (given a non-NULL @bs).
>> +     */
>> +
>> +    return bs;
>> +}
>> +
>> +/*
>> + * Return the first BDS that has not been added implicitly or that
>> + * does not have an RW-filtered child down the chain starting from @bs
>> + * (including @bs itself).
>> + */
>> +BlockDriverState *bdrv_skip_implicit_filters(BlockDriverState *bs)
>> +{
>> +    return bdrv_skip_filters(bs, true);
>> +}
>> +
>> +/*
>> + * Return the first BDS that does not have an RW-filtered child down
>> + * the chain starting from @bs (including @bs itself).
>> + */
>> +BlockDriverState *bdrv_skip_rw_filters(BlockDriverState *bs)
>> +{
>> +    return bdrv_skip_filters(bs, false);
>> +}
>> +
>> +/*
>> + * For a backing chain, return the first non-filter backing image.
> 
> or second, if we start from filter

Hm, in a sense.  Maybe:

> For a backing chain, return the first non-filter backing image of the
> first non-filter image.

?

>> + */
>> +BlockDriverState *bdrv_backing_chain_next(BlockDriverState *bs)
>> +{
>> +    return bdrv_skip_rw_filters(bdrv_filtered_cow_bs(bdrv_skip_rw_filters(bs)));
>> +}
> 
> 
> 



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