bdrv_co_block_status_above has several problems with handling short
backing files:
1. With want_zeros=true, it may return ret with BDRV_BLOCK_ZERO but
without BDRV_BLOCK_ALLOCATED flag, when actually short backing file
which produces these after-EOF zeros is inside requested backing
sequence.
2. With want_zero=false, it may return pnum=0 prior to actual EOF,
because of EOF of short backing file.
Fix these things, making logic about short backing files clearer.
Note that 154 output changed, because now bdrv_block_status_above don't
merge unallocated zeros with zeros after EOF (which are actually
"allocated" in POV of read from backing-chain top) and is_zero() just
don't understand that the whole head or tail is zero. We may update
is_zero to call bdrv_block_status_above several times, or add flag to
bdrv_block_status_above that we are not interested in ALLOCATED flag,
so ranges with different ALLOCATED status may be merged, but actually,
it seems that we'd better don't care about this corner case.
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
block/io.c | 38 +++++++++++++++++++++++++++++---------
tests/qemu-iotests/154.out | 4 ++--
2 files changed, 31 insertions(+), 11 deletions(-)
diff --git a/block/io.c b/block/io.c
index 121ce17a49..db990e812b 100644
--- a/block/io.c
+++ b/block/io.c
@@ -2461,25 +2461,45 @@ static int coroutine_fn bdrv_co_block_status_above(BlockDriverState *bs,
ret = bdrv_co_block_status(p, want_zero, offset, bytes, pnum, map,
file);
if (ret < 0) {
- break;
+ return ret;
}
- if (ret & BDRV_BLOCK_ZERO && ret & BDRV_BLOCK_EOF && !first) {
+ if (*pnum == 0) {
+ if (first) {
+ return ret;
+ }
+
/*
- * Reading beyond the end of the file continues to read
- * zeroes, but we can only widen the result to the
- * unallocated length we learned from an earlier
- * iteration.
+ * Reads from bs for the selected region will return zeroes,
+ * produced because the current level is short. We should consider
+ * it as allocated.
+ *
+ * TODO: Should we report p as file here?
*/
+ assert(ret & BDRV_BLOCK_EOF);
*pnum = bytes;
+ return BDRV_BLOCK_ZERO | BDRV_BLOCK_ALLOCATED;
}
- if (ret & (BDRV_BLOCK_ZERO | BDRV_BLOCK_DATA)) {
- break;
+ if (ret & BDRV_BLOCK_ALLOCATED) {
+ /* We've found the node and the status, we must return. */
+
+ if (ret & BDRV_BLOCK_ZERO && ret & BDRV_BLOCK_EOF && !first) {
+ /*
+ * This level is also responsible for reads after EOF inside
+ * the unallocated region in the previous level.
+ */
+ *pnum = bytes;
+ }
+
+ return ret;
}
+
/* [offset, pnum] unallocated on this layer, which could be only
* the first part of [offset, bytes]. */
- bytes = MIN(bytes, *pnum);
+ assert(*pnum <= bytes);
+ bytes = *pnum;
first = false;
}
+
return ret;
}
diff --git a/tests/qemu-iotests/154.out b/tests/qemu-iotests/154.out
index fa3673317f..a203dfcadd 100644
--- a/tests/qemu-iotests/154.out
+++ b/tests/qemu-iotests/154.out
@@ -310,13 +310,13 @@ wrote 512/512 bytes at offset 134217728
512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
2048/2048 bytes allocated at offset 128 MiB
[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
-{ "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
+{ "start": 134217728, "length": 2048, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
wrote 512/512 bytes at offset 134219264
512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
2048/2048 bytes allocated at offset 128 MiB
[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
-{ "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
+{ "start": 134217728, "length": 2048, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
wrote 1024/1024 bytes at offset 134218240
1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
--
2.21.0
On 5/19/20 2:54 PM, Vladimir Sementsov-Ogievskiy wrote:
> bdrv_co_block_status_above has several problems with handling short
> backing files:
>
> 1. With want_zeros=true, it may return ret with BDRV_BLOCK_ZERO but
> without BDRV_BLOCK_ALLOCATED flag, when actually short backing file
> which produces these after-EOF zeros is inside requested backing
> sequence.
That's intentional. That portion of the guest-visible data reads as
zero (BDRV_BLOCK_ZERO set) but was NOT read from the top layer, but
rather synthesized by the block layer because it derived from the
backing file but was beyond EOF of that backing layer
(BDRV_BLOCK_ALLOCATED is clear).
>
> 2. With want_zero=false, it may return pnum=0 prior to actual EOF,
> because of EOF of short backing file.
Do you have a reproducer for this? In my experience, this is not
possible. Generally, if you request status that overlaps EOF of the
backing, you get a response truncated to the end of the backing, and you
are then likely to follow up with a subsequent status request starting
from the underlying EOF which then sees the desired unallocated zeroes:
back xxxx
top yy------
request ^^^^^^
response ^^
request ^^^^
response ^^^^
>
> Fix these things, making logic about short backing files clearer.
>
> Note that 154 output changed, because now bdrv_block_status_above don't
doesn't
> merge unallocated zeros with zeros after EOF (which are actually
> "allocated" in POV of read from backing-chain top) and is_zero() just
> don't understand that the whole head or tail is zero. We may update
> is_zero to call bdrv_block_status_above several times, or add flag to
> bdrv_block_status_above that we are not interested in ALLOCATED flag,
> so ranges with different ALLOCATED status may be merged, but actually,
> it seems that we'd better don't care about this corner case.
This actually sounds like an avoidable regression. :(
I argue that if we did not explicitly write data/zero clusters in the
tail of the top layer, then those clusters are not allocated from the
POV of reading from the backing-chain top. Yes, we know what their
contents will be, but we also know what the contents of unallocated
clusters will be when there is no backing file at all - basically, after
your other patch series to drop unallocated_blocks_are_zero:
https://lists.gnu.org/archive/html/qemu-devel/2020-05/msg05429.html
then we know that only format drivers that can support backing files
even care what allocation means, and 'allocated' strictly means that the
data comes from the top layer rather than from a backing (whether
directly from the backing, or synthesized as zero by the block layer
because it was beyond EOF of the backing).
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
> block/io.c | 38 +++++++++++++++++++++++++++++---------
> tests/qemu-iotests/154.out | 4 ++--
> 2 files changed, 31 insertions(+), 11 deletions(-)
>
I'm already not a fan of this patch - it adds lines rather than removes,
and seems to add a regression.
> diff --git a/block/io.c b/block/io.c
> index 121ce17a49..db990e812b 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -2461,25 +2461,45 @@ static int coroutine_fn bdrv_co_block_status_above(BlockDriverState *bs,
> ret = bdrv_co_block_status(p, want_zero, offset, bytes, pnum, map,
> file);
> if (ret < 0) {
> - break;
> + return ret;
> }
> - if (ret & BDRV_BLOCK_ZERO && ret & BDRV_BLOCK_EOF && !first) {
> + if (*pnum == 0) {
> + if (first) {
> + return ret;
> + }
> +
> /*
> - * Reading beyond the end of the file continues to read
> - * zeroes, but we can only widen the result to the
> - * unallocated length we learned from an earlier
> - * iteration.
> + * Reads from bs for the selected region will return zeroes,
> + * produced because the current level is short. We should consider
> + * it as allocated.
Why? If we replaced the backing file to something longer (qemu-img
rebase -u), we would WANT to read from the backing file. The only
reason we read zero is because the block layer synthesized it _while_
deferring to the backing layer, not because it was directly allocated in
the top layer.
> + *
> + * TODO: Should we report p as file here?
No. Reporting 'file' only makes sense if you can point to an offset
within that file that would read the guest-visible data in question -
but when the data is synthesized, there is no such offset.
> */
> + assert(ret & BDRV_BLOCK_EOF);
> *pnum = bytes;
> + return BDRV_BLOCK_ZERO | BDRV_BLOCK_ALLOCATED;
> }
> - if (ret & (BDRV_BLOCK_ZERO | BDRV_BLOCK_DATA)) {
> - break;
> + if (ret & BDRV_BLOCK_ALLOCATED) {
> + /* We've found the node and the status, we must return. */
> +
> + if (ret & BDRV_BLOCK_ZERO && ret & BDRV_BLOCK_EOF && !first) {
> + /*
> + * This level is also responsible for reads after EOF inside
> + * the unallocated region in the previous level.
> + */
> + *pnum = bytes;
> + }
> +
> + return ret;
> }
> +
> /* [offset, pnum] unallocated on this layer, which could be only
> * the first part of [offset, bytes]. */
> - bytes = MIN(bytes, *pnum);
> + assert(*pnum <= bytes);
> + bytes = *pnum;
> first = false;
> }
> +
> return ret;
> }
>
> diff --git a/tests/qemu-iotests/154.out b/tests/qemu-iotests/154.out
> index fa3673317f..a203dfcadd 100644
> --- a/tests/qemu-iotests/154.out
> +++ b/tests/qemu-iotests/154.out
> @@ -310,13 +310,13 @@ wrote 512/512 bytes at offset 134217728
> 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
> 2048/2048 bytes allocated at offset 128 MiB
> [{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
> -{ "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
> +{ "start": 134217728, "length": 2048, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
The fact that we no longer see zeroes in the tail of the file makes me
think this patch is wrong.
> Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
> wrote 512/512 bytes at offset 134219264
> 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
> 2048/2048 bytes allocated at offset 128 MiB
> [{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
> -{ "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
> +{ "start": 134217728, "length": 2048, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
> Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
> wrote 1024/1024 bytes at offset 134218240
> 1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
>
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
19.05.2020 23:41, Eric Blake wrote:
> On 5/19/20 2:54 PM, Vladimir Sementsov-Ogievskiy wrote:
>> bdrv_co_block_status_above has several problems with handling short
>> backing files:
>>
>> 1. With want_zeros=true, it may return ret with BDRV_BLOCK_ZERO but
>> without BDRV_BLOCK_ALLOCATED flag, when actually short backing file
>> which produces these after-EOF zeros is inside requested backing
>> sequence.
>
> That's intentional. That portion of the guest-visible data reads as zero (BDRV_BLOCK_ZERO set) but was NOT read from the top layer, but rather synthesized by the block layer because it derived from the backing file but was beyond EOF of that backing layer (BDRV_BLOCK_ALLOCATED is clear).
Not in top yes. But _inside_ the requested base..top backing-chain-part. So it should be considered ALLOCATED, as we should not go to further backing.
Assume the following chain:
top aa--
middle bb
base xxxx
(so, middle is short)
block_status(top, 2) should return ZERO without ALLOCATED, as yes it's ZERO and yes, it's from another layer
block_status_above(top, base, 2) should return ZERO with ALLOCATED, as it's ZERO, and it's produced inside requested backing-chain-region, actually, it's produced because of short middle node. We must report ALLOCATED to show that we are not going to read from base.
>
>>
>> 2. With want_zero=false, it may return pnum=0 prior to actual EOF,
>> because of EOF of short backing file.
>
> Do you have a reproducer for this?
No, I don't have one, but it seems possible at least with want_zero=false. I'll think of it tomorrow, too tired now.
> In my experience, this is not possible. Generally, if you request status that overlaps EOF of the backing, you get a response truncated to the end of the backing, and you are then likely to follow up with a subsequent status request starting from the underlying EOF which then sees the desired unallocated zeroes:
>
> back xxxx
> top yy------
> request ^^^^^^
> response ^^
> request ^^^^
> response ^^^^
>
>>
>> Fix these things, making logic about short backing files clearer.
>>
>> Note that 154 output changed, because now bdrv_block_status_above don't
>
> doesn't
>
>> merge unallocated zeros with zeros after EOF (which are actually
>> "allocated" in POV of read from backing-chain top) and is_zero() just
>> don't understand that the whole head or tail is zero. We may update
>> is_zero to call bdrv_block_status_above several times, or add flag to
>> bdrv_block_status_above that we are not interested in ALLOCATED flag,
>> so ranges with different ALLOCATED status may be merged, but actually,
>> it seems that we'd better don't care about this corner case.
>
> This actually sounds like an avoidable regression. :(
I don't see real problem in it. But it seems not hard to avoid it, so I will try to.
>
> I argue that if we did not explicitly write data/zero clusters in the tail of the top layer, then those clusters are not allocated from the POV of reading from the backing-chain top. Yes, we know what their contents will be, but we also know what the contents of unallocated clusters will be when there is no backing file at all - basically, after your other patch series to drop unallocated_blocks_are_zero:
> https://lists.gnu.org/archive/html/qemu-devel/2020-05/msg05429.html
> then we know that only format drivers that can support backing files even care what allocation means, and 'allocated' strictly means that the data comes from the top layer rather than from a backing (whether directly from the backing, or synthesized as zero by the block layer because it was beyond EOF of the backing).
I agree about allocated in top, returned by block_status. But this patch is for allocated_above, and the ALLOCATED status is not about top, but about a set of nodes from base (not inclusive) to top.
>
>>
>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>> ---
>> block/io.c | 38 +++++++++++++++++++++++++++++---------
>> tests/qemu-iotests/154.out | 4 ++--
>> 2 files changed, 31 insertions(+), 11 deletions(-)
>>
>
> I'm already not a fan of this patch - it adds lines rather than removes, and seems to add a regression.
>
>> diff --git a/block/io.c b/block/io.c
>> index 121ce17a49..db990e812b 100644
>> --- a/block/io.c
>> +++ b/block/io.c
>> @@ -2461,25 +2461,45 @@ static int coroutine_fn bdrv_co_block_status_above(BlockDriverState *bs,
>> ret = bdrv_co_block_status(p, want_zero, offset, bytes, pnum, map,
>> file);
>> if (ret < 0) {
>> - break;
>> + return ret;
>> }
>> - if (ret & BDRV_BLOCK_ZERO && ret & BDRV_BLOCK_EOF && !first) {
>> + if (*pnum == 0) {
>> + if (first) {
>> + return ret;
>> + }
>> +
>> /*
>> - * Reading beyond the end of the file continues to read
>> - * zeroes, but we can only widen the result to the
>> - * unallocated length we learned from an earlier
>> - * iteration.
>> + * Reads from bs for the selected region will return zeroes,
>> + * produced because the current level is short. We should consider
>> + * it as allocated.
>
> Why? If we replaced the backing file to something longer (qemu-img rebase -u), we would WANT to read from the backing file. The only reason we read zero is because the block layer synthesized it _while_ deferring to the backing layer, not because it was directly allocated in the top layer.
No, if we replace backing file of the current layer, nothing will change, as _this_ layer is short, not the backing. Or which backing file do you mean? If you mean current bs, than replacing it doesn't make sense in the context, as block_status_above requested the current bs (as part of base..top range), not the other one.
>
>> + *
>> + * TODO: Should we report p as file here?
>
> No. Reporting 'file' only makes sense if you can point to an offset within that file that would read the guest-visible data in question - but when the data is synthesized, there is no such offset.
I don't know. It still adds some information about which level is responsible for these ZEROES. Kevin argued that it make sense.
>
>> */
>> + assert(ret & BDRV_BLOCK_EOF);
>> *pnum = bytes;
>> + return BDRV_BLOCK_ZERO | BDRV_BLOCK_ALLOCATED;
>> }
>> - if (ret & (BDRV_BLOCK_ZERO | BDRV_BLOCK_DATA)) {
>> - break;
>> + if (ret & BDRV_BLOCK_ALLOCATED) {
>> + /* We've found the node and the status, we must return. */
>> +
>> + if (ret & BDRV_BLOCK_ZERO && ret & BDRV_BLOCK_EOF && !first) {
>> + /*
>> + * This level is also responsible for reads after EOF inside
>> + * the unallocated region in the previous level.
>> + */
>> + *pnum = bytes;
>> + }
>> +
>> + return ret;
>> }
>> +
>> /* [offset, pnum] unallocated on this layer, which could be only
>> * the first part of [offset, bytes]. */
>> - bytes = MIN(bytes, *pnum);
>> + assert(*pnum <= bytes);
>> + bytes = *pnum;
>> first = false;
>> }
>> +
>> return ret;
>> }
>> diff --git a/tests/qemu-iotests/154.out b/tests/qemu-iotests/154.out
>> index fa3673317f..a203dfcadd 100644
>> --- a/tests/qemu-iotests/154.out
>> +++ b/tests/qemu-iotests/154.out
>> @@ -310,13 +310,13 @@ wrote 512/512 bytes at offset 134217728
>> 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
>> 2048/2048 bytes allocated at offset 128 MiB
>> [{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
>> -{ "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
>> +{ "start": 134217728, "length": 2048, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
>
> The fact that we no longer see zeroes in the tail of the file makes me think this patch is wrong.
>
>> Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
>> wrote 512/512 bytes at offset 134219264
>> 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
>> 2048/2048 bytes allocated at offset 128 MiB
>> [{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
>> -{ "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
>> +{ "start": 134217728, "length": 2048, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
>> Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
>> wrote 1024/1024 bytes at offset 134218240
>> 1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
>>
>
--
Best regards,
Vladimir
On 5/19/20 4:13 PM, Vladimir Sementsov-Ogievskiy wrote:
> 19.05.2020 23:41, Eric Blake wrote:
>> On 5/19/20 2:54 PM, Vladimir Sementsov-Ogievskiy wrote:
>>> bdrv_co_block_status_above has several problems with handling short
>>> backing files:
>>>
>>> 1. With want_zeros=true, it may return ret with BDRV_BLOCK_ZERO but
>>> without BDRV_BLOCK_ALLOCATED flag, when actually short backing file
>>> which produces these after-EOF zeros is inside requested backing
>>> sequence.
>>
>> That's intentional. That portion of the guest-visible data reads as
>> zero (BDRV_BLOCK_ZERO set) but was NOT read from the top layer, but
>> rather synthesized by the block layer because it derived from the
>> backing file but was beyond EOF of that backing layer
>> (BDRV_BLOCK_ALLOCATED is clear).
>
> Not in top yes. But _inside_ the requested base..top backing-chain-part.
> So it should be considered ALLOCATED, as we should not go to further
> backing.
Yes, I think I figured that out by patch 5.
>
> Assume the following chain:
>
> top aa--
> middle bb
> base xxxx
>
> (so, middle is short)
>
> block_status(top, 2) should return ZERO without ALLOCATED, as yes it's
> ZERO and yes, it's from another layer
>
> block_status_above(top, base, 2) should return ZERO with ALLOCATED, as
> it's ZERO, and it's produced inside requested backing-chain-region,
> actually, it's produced because of short middle node. We must report
> ALLOCATED to show that we are not going to read from base.
Yes, that matches my intuition. allocated_above says "where in the
chain did we get the data, since it did not come from top", and the
correct answer is "we got it from middle, due to synthesizing zero
beyond EOF". Okay, with that understanding in place, maybe this patch
is right. But I'll have to revisit it tomorrow on a fresh mind (it's
too late in the day for me to be sure that I'm getting it all straight
right now).
>
>>
>>>
>>> 2. With want_zero=false, it may return pnum=0 prior to actual EOF,
>>> because of EOF of short backing file.
>>
>> Do you have a reproducer for this?
>
> No, I don't have one, but it seems possible at least with
> want_zero=false. I'll think of it tomorrow, too tired now.
>
>> In my experience, this is not possible. Generally, if you request
>> status that overlaps EOF of the backing, you get a response truncated
>> to the end of the backing, and you are then likely to follow up with a
>> subsequent status request starting from the underlying EOF which then
>> sees the desired unallocated zeroes:
>>
>> back xxxx
>> top yy------
>> request ^^^^^^
>> response ^^
>> request ^^^^
>> response ^^^^
If we can come up with a reproducer where allocated_above returns
pnum=0, that would indeed prove my initial hesitation wrong (perhaps by:
back xxxxxxxx
mid1 xxxxxx
mid2 xxxx
mid3 xxxxxx
top xxxxxxxx
for various different start and base points within the chain?)
>>
>>>
>>> Fix these things, making logic about short backing files clearer.
>>>
>>> Note that 154 output changed, because now bdrv_block_status_above don't
>>
>> doesn't
>>
>>> merge unallocated zeros with zeros after EOF (which are actually
>>> "allocated" in POV of read from backing-chain top) and is_zero() just
>>> don't understand that the whole head or tail is zero. We may update
>>> is_zero to call bdrv_block_status_above several times, or add flag to
>>> bdrv_block_status_above that we are not interested in ALLOCATED flag,
>>> so ranges with different ALLOCATED status may be merged, but actually,
>>> it seems that we'd better don't care about this corner case.
>>
>> This actually sounds like an avoidable regression. :(
>
> I don't see real problem in it. But it seems not hard to avoid it, so I
> will try to.
I guess my real reasoning is: "I spent a lot of time trying to tweak
that test to not lose the fact that the tail of the image reads as
zero", because it looks weird if we later resize the image but still
have a glitch in the middle reporting one non-zero cluster out of a
larger range all because of the shenanigans that occurred around the
tail prior to resizing.
>>> +++ b/block/io.c
>>> @@ -2461,25 +2461,45 @@ static int coroutine_fn
>>> bdrv_co_block_status_above(BlockDriverState *bs,
>>> ret = bdrv_co_block_status(p, want_zero, offset, bytes,
>>> pnum, map,
>>> file);
>>> if (ret < 0) {
>>> - break;
>>> + return ret;
>>> }
>>> - if (ret & BDRV_BLOCK_ZERO && ret & BDRV_BLOCK_EOF && !first) {
>>> + if (*pnum == 0) {
>>> + if (first) {
>>> + return ret;
>>> + }
>>> +
>>> /*
>>> - * Reading beyond the end of the file continues to read
>>> - * zeroes, but we can only widen the result to the
>>> - * unallocated length we learned from an earlier
>>> - * iteration.
>>> + * Reads from bs for the selected region will return
>>> zeroes,
>>> + * produced because the current level is short. We
>>> should consider
>>> + * it as allocated.
>>
>> Why? If we replaced the backing file to something longer (qemu-img
>> rebase -u), we would WANT to read from the backing file. The only
>> reason we read zero is because the block layer synthesized it _while_
>> deferring to the backing layer, not because it was directly allocated
>> in the top layer.
>
> No, if we replace backing file of the current layer, nothing will
> change, as _this_ layer is short, not the backing. Or which backing file
> do you mean? If you mean current bs, than replacing it doesn't make
> sense in the context, as block_status_above requested the current bs (as
> part of base..top range), not the other one.
Maybe it's just the comment wording that needs help. After reading
through patch 5, it looks like my problem is now coming up with a
comment to the effect of "the top layer deferred to this layer, and
because this layer is short, any zeroes that we synthesize beyond EOF
behave as if they were allocated at this layer".
>
>>
>>> + *
>>> + * TODO: Should we report p as file here?
>>
>> No. Reporting 'file' only makes sense if you can point to an offset
>> within that file that would read the guest-visible data in question -
>> but when the data is synthesized, there is no such offset.
>
> I don't know. It still adds some information about which level is
> responsible for these ZEROES. Kevin argued that it make sense.
It took me a while, but I'm coming around to it: my initial read was
assuming that you were reporting that the tail was being claimed as
allocated by top; but in reality, you are fixing things to claim it as
being allocated by mid. The former is wrong (top did not allocate, it
deferred to mid); but the latter does indeed make sense (reading from
mid ended up synthesizing, which means that our hunt for the data ends
at mid and we never traverse deeper, regardless of whether base may also
have data). But now it's a question of whether the code matches that
textual description, and I'm a bit too fried to answer that question
properly today :)
>>> +++ b/tests/qemu-iotests/154.out
>>> @@ -310,13 +310,13 @@ wrote 512/512 bytes at offset 134217728
>>> 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
>>> 2048/2048 bytes allocated at offset 128 MiB
>>> [{ "start": 0, "length": 134217728, "depth": 1, "zero": true,
>>> "data": false},
>>> -{ "start": 134217728, "length": 2048, "depth": 0, "zero": true,
>>> "data": false}]
>>> +{ "start": 134217728, "length": 2048, "depth": 0, "zero": false,
>>> "data": true, "offset": OFFSET}]
>>
>> The fact that we no longer see zeroes in the tail of the file makes me
>> think this patch is wrong.
So, if we can avoid that minor regression, and still otherwise report
zeroes as allocated from mid, then I think we'll be on the right track.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
20.05.2020 00:48, Eric Blake wrote:
> On 5/19/20 4:13 PM, Vladimir Sementsov-Ogievskiy wrote:
>> 19.05.2020 23:41, Eric Blake wrote:
>>> On 5/19/20 2:54 PM, Vladimir Sementsov-Ogievskiy wrote:
>>>> bdrv_co_block_status_above has several problems with handling short
>>>> backing files:
>>>>
>>>> 1. With want_zeros=true, it may return ret with BDRV_BLOCK_ZERO but
>>>> without BDRV_BLOCK_ALLOCATED flag, when actually short backing file
>>>> which produces these after-EOF zeros is inside requested backing
>>>> sequence.
>>>
>>> That's intentional. That portion of the guest-visible data reads as zero (BDRV_BLOCK_ZERO set) but was NOT read from the top layer, but rather synthesized by the block layer because it derived from the backing file but was beyond EOF of that backing layer (BDRV_BLOCK_ALLOCATED is clear).
>>
>> Not in top yes. But _inside_ the requested base..top backing-chain-part. So it should be considered ALLOCATED, as we should not go to further backing.
>
> Yes, I think I figured that out by patch 5.
>
>>
>> Assume the following chain:
>>
>> top aa--
>> middle bb
>> base xxxx
>>
>> (so, middle is short)
>>
>> block_status(top, 2) should return ZERO without ALLOCATED, as yes it's ZERO and yes, it's from another layer
>>
>> block_status_above(top, base, 2) should return ZERO with ALLOCATED, as it's ZERO, and it's produced inside requested backing-chain-region, actually, it's produced because of short middle node. We must report ALLOCATED to show that we are not going to read from base.
>
> Yes, that matches my intuition. allocated_above says "where in the chain did we get the data, since it did not come from top", and the correct answer is "we got it from middle, due to synthesizing zero beyond EOF". Okay, with that understanding in place, maybe this patch is right. But I'll have to revisit it tomorrow on a fresh mind (it's too late in the day for me to be sure that I'm getting it all straight right now).
>
>>
>>>
>>>>
>>>> 2. With want_zero=false, it may return pnum=0 prior to actual EOF,
>>>> because of EOF of short backing file.
>>>
>>> Do you have a reproducer for this?
>>
>> No, I don't have one, but it seems possible at least with want_zero=false. I'll think of it tomorrow, too tired now.
>>
>>> In my experience, this is not possible. Generally, if you request status that overlaps EOF of the backing, you get a response truncated to the end of the backing, and you are then likely to follow up with a subsequent status request starting from the underlying EOF which then sees the desired unallocated zeroes:
>>>
>>> back xxxx
>>> top yy------
>>> request ^^^^^^
>>> response ^^
>>> request ^^^^
>>> response ^^^^
>
> If we can come up with a reproducer where allocated_above returns pnum=0, that would indeed prove my initial hesitation wrong (perhaps by:
>
> back xxxxxxxx
> mid1 xxxxxx
> mid2 xxxx
> mid3 xxxxxx
> top xxxxxxxx
>
> for various different start and base points within the chain?)
Seems, we just don't have users of bdrv_co_block_status_above with base points within the chain, base is always NULL or backing of top. So, I don't think we have a reproducer.
>
>>>
>>>>
>>>> Fix these things, making logic about short backing files clearer.
>>>>
>>>> Note that 154 output changed, because now bdrv_block_status_above don't
>>>
>>> doesn't
>>>
>>>> merge unallocated zeros with zeros after EOF (which are actually
>>>> "allocated" in POV of read from backing-chain top) and is_zero() just
>>>> don't understand that the whole head or tail is zero. We may update
>>>> is_zero to call bdrv_block_status_above several times, or add flag to
>>>> bdrv_block_status_above that we are not interested in ALLOCATED flag,
>>>> so ranges with different ALLOCATED status may be merged, but actually,
>>>> it seems that we'd better don't care about this corner case.
>>>
>>> This actually sounds like an avoidable regression. :(
>>
>> I don't see real problem in it. But it seems not hard to avoid it, so I will try to.
>
> I guess my real reasoning is: "I spent a lot of time trying to tweak that test to not lose the fact that the tail of the image reads as zero", because it looks weird if we later resize the image but still have a glitch in the middle reporting one non-zero cluster out of a larger range all because of the shenanigans that occurred around the tail prior to resizing.
>
>>>> +++ b/block/io.c
>>>> @@ -2461,25 +2461,45 @@ static int coroutine_fn bdrv_co_block_status_above(BlockDriverState *bs,
>>>> ret = bdrv_co_block_status(p, want_zero, offset, bytes, pnum, map,
>>>> file);
>>>> if (ret < 0) {
>>>> - break;
>>>> + return ret;
>>>> }
>>>> - if (ret & BDRV_BLOCK_ZERO && ret & BDRV_BLOCK_EOF && !first) {
>>>> + if (*pnum == 0) {
>>>> + if (first) {
>>>> + return ret;
>>>> + }
>>>> +
>>>> /*
>>>> - * Reading beyond the end of the file continues to read
>>>> - * zeroes, but we can only widen the result to the
>>>> - * unallocated length we learned from an earlier
>>>> - * iteration.
>>>> + * Reads from bs for the selected region will return zeroes,
>>>> + * produced because the current level is short. We should consider
>>>> + * it as allocated.
>>>
>>> Why? If we replaced the backing file to something longer (qemu-img rebase -u), we would WANT to read from the backing file. The only reason we read zero is because the block layer synthesized it _while_ deferring to the backing layer, not because it was directly allocated in the top layer.
>>
>> No, if we replace backing file of the current layer, nothing will change, as _this_ layer is short, not the backing. Or which backing file do you mean? If you mean current bs, than replacing it doesn't make sense in the context, as block_status_above requested the current bs (as part of base..top range), not the other one.
>
> Maybe it's just the comment wording that needs help. After reading through patch 5, it looks like my problem is now coming up with a comment to the effect of "the top layer deferred to this layer, and because this layer is short, any zeroes that we synthesize beyond EOF behave as if they were allocated at this layer".
>
>>
>>>
>>>> + *
>>>> + * TODO: Should we report p as file here?
>>>
>>> No. Reporting 'file' only makes sense if you can point to an offset within that file that would read the guest-visible data in question - but when the data is synthesized, there is no such offset.
>>
>> I don't know. It still adds some information about which level is responsible for these ZEROES. Kevin argued that it make sense.
>
> It took me a while, but I'm coming around to it: my initial read was assuming that you were reporting that the tail was being claimed as allocated by top; but in reality, you are fixing things to claim it as being allocated by mid. The former is wrong (top did not allocate, it deferred to mid); but the latter does indeed make sense (reading from mid ended up synthesizing, which means that our hunt for the data ends at mid and we never traverse deeper, regardless of whether base may also have data). But now it's a question of whether the code matches that textual description, and I'm a bit too fried to answer that question properly today :)
>
>>>> +++ b/tests/qemu-iotests/154.out
>>>> @@ -310,13 +310,13 @@ wrote 512/512 bytes at offset 134217728
>>>> 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
>>>> 2048/2048 bytes allocated at offset 128 MiB
>>>> [{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
>>>> -{ "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
>>>> +{ "start": 134217728, "length": 2048, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
>>>
>>> The fact that we no longer see zeroes in the tail of the file makes me think this patch is wrong.
>
> So, if we can avoid that minor regression, and still otherwise report zeroes as allocated from mid, then I think we'll be on the right track.
>
--
Best regards,
Vladimir
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