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Re: [Qemu-block] [PATCH v5 15/42] block: Re-evaluate backing file handli
From: |
Vladimir Sementsov-Ogievskiy |
Subject: |
Re: [Qemu-block] [PATCH v5 15/42] block: Re-evaluate backing file handling in reopen |
Date: |
Fri, 14 Jun 2019 16:43:55 +0000 |
14.06.2019 18:52, Max Reitz wrote:
> On 14.06.19 15:42, Vladimir Sementsov-Ogievskiy wrote:
>> 13.06.2019 1:09, Max Reitz wrote:
>>> Reopening a node's backing child needs a bit of special handling because
>>> the "backing" child has different defaults than all other children
>>> (among other things). Adding filter support here is a bit more
>>> difficult than just using the child access functions. In fact, we often
>>> have to directly use bs->backing because these functions are about the
>>> "backing" child (which may or may not be the COW backing file).
>>>
>>> Signed-off-by: Max Reitz <address@hidden>
>>> ---
>>> block.c | 36 +++++++++++++++++++++++++++++-------
>>> 1 file changed, 29 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/block.c b/block.c
>>> index 505b3e9a01..db2759c10d 100644
>>> --- a/block.c
>>> +++ b/block.c
>>> @@ -3542,17 +3542,39 @@ static int
>>> bdrv_reopen_parse_backing(BDRVReopenState *reopen_state,
>>> }
>>> }
>>>
>>> + /*
>>> + * Ensure that @bs can really handle backing files, because we are
>>> + * about to give it one (or swap the existing one)
>>> + */
>>> + if (bs->drv->is_filter) {
>>> + /* Filters always have a file or a backing child */
>>> + if (!bs->backing) {
>>> + error_setg(errp, "'%s' is a %s filter node that does not
>>> support a "
>>> + "backing child", bs->node_name,
>>> bs->drv->format_name);
>>> + return -EINVAL;
>>> + }
>>> + } else if (!bs->drv->supports_backing) {
>>> + error_setg(errp, "Driver '%s' of node '%s' does not support
>>> backing "
>>> + "files", bs->drv->format_name, bs->node_name);
>>> + return -EINVAL;
>>> + }
>>
>> hmm, shouldn't we have these checks for overlay_bs?
>
> I think this is correct here because this is the only node the user has
> control over, so this is the only one we can reasonably complain about.
>
> And I do think it is reasonable to complain about.
>
>>> +
>>> /*
>>> * 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 (bdrv_filtered_bs(overlay_bs) &&
>>> + bdrv_filtered_bs(overlay_bs)->implicit)
>>> + {
>>> + overlay_bs = bdrv_filtered_bs(overlay_bs);
>>> }
>>
>> here, overlay_bs may be some filter with file child ..
>>
>>>
>>> /* 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 != bdrv_filtered_bs(overlay_bs)) {
>>> /* Check for implicit nodes between bs and its backing file */
>>> if (bs != overlay_bs) {
>>> error_setg(errp, "Cannot change backing link if '%s' has "
>>> @@ -3560,8 +3582,8 @@ static int bdrv_reopen_parse_backing(BDRVReopenState
>>> *reopen_state,
>>> return -EPERM;
>>> }
>>> /* Check if the backing link that we want to replace is frozen */
>>> - if (bdrv_is_backing_chain_frozen(overlay_bs,
>>> backing_bs(overlay_bs),
>>> - errp)) {
>>> + if (bdrv_is_backing_chain_frozen(overlay_bs,
>>> + child_bs(overlay_bs->backing),
>>> errp)) {
>>
>> .. and here we are doing wrong thing, as it don't have backing child
>>
>> Aha, you use the fact that we now don't have implicit filters with file
>> child. Then, should
>> we add an assertion for this?
>
> No, that wasn’t my intention. The real reason is that all of this is a
> mess.
>
> Here is the full context:
>
>> overlay_bs = bs;
>> while (bdrv_filtered_bs(overlay_bs) &&
>> bdrv_filtered_bs(overlay_bs)->implicit)
>> {
>> overlay_bs = bdrv_filtered_bs(overlay_bs);
>> }
>>
>> /* If we want to replace the backing file we need some extra checks */
>> if (new_backing_bs != bdrv_filtered_bs(overlay_bs)) {
>> /* Check for implicit nodes between bs and its backing file */
>> if (bs != overlay_bs) {
>> error_setg(errp, "Cannot change backing link if '%s' has "
>> "an implicit backing file", bs->node_name);
>> return -EPERM;
>> }
>> /* Check if the backing link that we want to replace is frozen */
>> if (bdrv_is_backing_chain_frozen(overlay_bs,
>> child_bs(overlay_bs->backing),
>> errp)) {
>> return -EPERM;
>> }
>
> Note the “Check for implicit nodes” thing. If we get to the frozen
> check, we have already confirmed that overlay_bs == bs, so we then know
> that overlay_bs->backing works.
>
> I can add an additional comment to make that more clear. It took myself
> quite a bit of digging to figure that out again...
Aha, I see it. Comment would be good.
>
> (The reason for the loop is that we want to be able to recognize when
> the user tries to not change the backing file. In that case, we don’t
> have to do anything, but because the user doesn’t know about implicit
> nodes, we have to skip them in order to check whether the user actually
> doesn’t want to change anything.)
>
> Max
>
>>> return -EPERM;
>>> }
>>> reopen_state->replace_backing_bs = true;
>>> @@ -3712,7 +3734,7 @@ int bdrv_reopen_prepare(BDRVReopenState
>>> *reopen_state, BlockReopenQueue *queue,
>>> * its metadata. Otherwise the 'backing' option can be omitted.
>>> */
>>> if (drv->supports_backing && reopen_state->backing_missing &&
>>> - (backing_bs(reopen_state->bs) ||
>>> reopen_state->bs->backing_file[0])) {
>>> + (reopen_state->bs->backing || reopen_state->bs->backing_file[0])) {
>>> error_setg(errp, "backing is missing for '%s'",
>>> reopen_state->bs->node_name);
>>> ret = -EINVAL;
>>> @@ -3857,7 +3879,7 @@ void bdrv_reopen_commit(BDRVReopenState *reopen_state)
>>> * from bdrv_set_backing_hd()) has the new values.
>>> */
>>> if (reopen_state->replace_backing_bs) {
>>> - BlockDriverState *old_backing_bs = backing_bs(bs);
>>> + BlockDriverState *old_backing_bs = child_bs(bs->backing);
>>> assert(!old_backing_bs || !old_backing_bs->implicit);
>>> /* Abort the permission update on the backing bs we're detaching
>>> */
>>> if (old_backing_bs) {
>>>
>>
>>
>
>
--
Best regards,
Vladimir
- Re: [Qemu-block] [PATCH v5 13/42] block: Use CAFs in block status functions, (continued)
[Qemu-block] [PATCH v5 15/42] block: Re-evaluate backing file handling in reopen, Max Reitz, 2019/06/12
[Qemu-block] [PATCH v5 16/42] block: Use child access functions when flushing, Max Reitz, 2019/06/12
[Qemu-block] [PATCH v5 17/42] block: Use CAFs in bdrv_refresh_limits(), Max Reitz, 2019/06/12
[Qemu-block] [PATCH v5 18/42] block: Use CAFs in bdrv_refresh_filename(), Max Reitz, 2019/06/12
[Qemu-block] [PATCH v5 19/42] block: Use CAF in bdrv_co_rw_vmstate(), Max Reitz, 2019/06/12
[Qemu-block] [PATCH v5 20/42] block/snapshot: Fall back to storage child, Max Reitz, 2019/06/12