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Re: [Qemu-block] [PATCH v3 03/12] block: Filtered children access functi
From: |
Max Reitz |
Subject: |
Re: [Qemu-block] [PATCH v3 03/12] block: Filtered children access functions |
Date: |
Fri, 15 Feb 2019 17:45:38 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.0 |
On 14.02.19 03:25, Eric Blake wrote:
> On 2/13/19 4:53 PM, 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 <address@hidden>
>> ---
>
>> +++ b/qapi/block-core.json
>> @@ -2417,6 +2417,10 @@
>> # On successful completion the image file is updated to drop the backing
>> file
>> # and the BLOCK_JOB_COMPLETED event is emitted.
>> #
>> +# In case @device is a filter node, block-stream modifies the first
>> non-filter
>> +# overlay node below it to point to base's backing node (or NULL if @base
>> was
>> +# not specified) instead of modifying @device itself.
>
> Maybe s/In case/If/
Works for me.
>> +/*
>> + * For a backing chain, return the first non-filter backing image.
>> + */
>> +BlockDriverState *bdrv_backing_chain_next(BlockDriverState *bs)
>> +{
>> + return
>> bdrv_skip_rw_filters(bdrv_filtered_cow_bs(bdrv_skip_rw_filters(bs)));
>
> Quite a mouthful, but looks correct.
>
>
>> +++ b/block/io.c
>> @@ -118,8 +118,17 @@ static void bdrv_merge_limits(BlockLimits *dst, const
>> BlockLimits *src)
>> void bdrv_refresh_limits(BlockDriverState *bs, Error **errp)
>> {
>> BlockDriver *drv = bs->drv;
>> + BlockDriverState *storage_bs;
>> + BlockDriverState *cow_bs = bdrv_filtered_cow_bs(bs);
>
> If the backing file is filtered by a blkdebug layer that intentionally
> is trying to advertise alternative block sizes...
>
>> Error *local_err = NULL;
>>
>> + /*
>> + * FIXME: There should be a function for this, and in fact there
>> + * will be as of a follow-up patch.
>> + */
>> + storage_bs =
>> + child_bs(bs->file) ?: bdrv_filtered_rw_bs(bs);
>> +
>> memset(&bs->bl, 0, sizeof(bs->bl));
>>
>> if (!drv) {
>> @@ -131,13 +140,13 @@ void bdrv_refresh_limits(BlockDriverState *bs, Error
>> **errp)
>> drv->bdrv_aio_preadv) ? 1 : 512;
>>
>> /* Take some limits from the children as a default */
>> - if (bs->file) {
>> - bdrv_refresh_limits(bs->file->bs, &local_err);
>> + if (storage_bs) {
>> + bdrv_refresh_limits(storage_bs, &local_err);
>> if (local_err) {
>> error_propagate(errp, local_err);
>> return;
>> }
>> - bdrv_merge_limits(&bs->bl, &bs->file->bs->bl);
>> + bdrv_merge_limits(&bs->bl, &storage_bs->bl);
>> } else {
>> bs->bl.min_mem_alignment = 512;
>> bs->bl.opt_mem_alignment = getpagesize();
>> @@ -146,13 +155,13 @@ void bdrv_refresh_limits(BlockDriverState *bs, Error
>> **errp)
>> bs->bl.max_iov = IOV_MAX;
>> }
>>
>> - if (bs->backing) {
>> - bdrv_refresh_limits(bs->backing->bs, &local_err);
>> + if (cow_bs) {
>> + bdrv_refresh_limits(cow_bs, &local_err);
>
> ...then this change means that the active layer no longer picks up the
> blkdebug block sizes, but the original COW layer sizes.
How so?
I don't see how this changes current behavior with just COW/blkdebug
nodes at all. For blkdebug, bs->file is the storage_bs. For COW nodes,
bs->file is the storage_bs and bs->backing is the cow_bs. So these code
changes do nothing in that case.
(They only change something for filter nodes that use bs->backing for
their filtered node; because that is translated to storage_bs now.)
Note that bdrv_filtered_cow_bs() does not skip filters (i.e. the
blkdebug node). That would be bdrv_backing_chain_next().
> Is that
> intentional? I don't think it is fatal to the patch (as blkdebug is not
> used in production, but only in testing), but it may cause some
> head-scratching when trying to test behaviors of a COW child with
> different block sizes than the active layer by using blkdebug on top of
> the COW child. I guess I'll find out soon enough (on my todo list is
> fixing NBD to never split an NBD_CMD_READ or NBD_CMD_BLOCK_STATUS reply
> below the granularity advertised at the active layer, even if the
> backing file has a smaller granularity - and using blkdebug to force the
> backing file granularity was my original plan of attack - but it is not
> until this patch is applied that NBD can even locate the bitmap in a
> backing file when a filter is interposed)
>
> Reviewed-by: Eric Blake <address@hidden>
Thanks!
Max
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- [Qemu-block] [PATCH v3 08/12] block: Leave BDS.backing_file constant, (continued)
- [Qemu-block] [PATCH v3 08/12] block: Leave BDS.backing_file constant, Max Reitz, 2019/02/13
- [Qemu-block] [PATCH v3 10/12] iotests: Add filter mirror test cases, Max Reitz, 2019/02/13
- [Qemu-block] [PATCH v3 06/12] block: Fix check_to_replace_node(), Max Reitz, 2019/02/13
- [Qemu-block] [PATCH v3 05/12] block: Inline bdrv_co_block_status_from_*(), Max Reitz, 2019/02/13
- [Qemu-block] [PATCH v3 04/12] block: Storage child access function, Max Reitz, 2019/02/13
- [Qemu-block] [PATCH v3 01/12] block: Mark commit and mirror as filter drivers, Max Reitz, 2019/02/13
- [Qemu-block] [PATCH v3 03/12] block: Filtered children access functions, Max Reitz, 2019/02/13
- [Qemu-block] [PATCH v3 02/12] blockdev: Check @replaces in blockdev_mirror_common, Max Reitz, 2019/02/13