[PATCH 2/2] block/preallocate: fix image truncation logic

[PATCH 2/2] block/preallocate: fix image truncation logic

[Denis V. Lunev via]

Recent QEMU changes around preallocate_set_perm mandates that it is not
possible to poll on aio_context inside this function anymore. Thus
truncate operation has been moved inside bottom half. This bottom half
is scheduled from preallocate_set_perm() and that is all.

This approach proven to be problematic in a lot of places once
additional operations are executed over preallocate filter in
production. The code validates that permissions have been really changed
just after the call to the set operation.

All permissions operations or block driver graph changes are performed
inside the quiscent state in terms of the block layer. This means that
there are no in-flight packets which is guaranteed by the passing
through bdrv_drain() section.

The idea is that we should effectively disable preallocate filter inside
bdrv_drain() and unblock permission changes. This section is definitely
not on the hot path and additional single truncate operation will not
hurt.

Unfortunately bdrv_drain_begin() callback according to the documentation
also disallow waiting inside. Thus original approach with the bottom
half is not changed. bdrv_drain_begin() schedules the operation and in
order to ensure that it has been really executed before completion of
the section increments the amount of in-flight requests.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
CC: Kevin Wolf <kwolf@redhat.com>
---
 block/preallocate.c    | 38 ++++++++++++++++++++++++++++++++++----
 tests/qemu-iotests/298 |  6 ++++--
 2 files changed, 38 insertions(+), 6 deletions(-)

diff --git a/block/preallocate.c b/block/preallocate.c
index 1cf854966c..d78ef0b045 100644
--- a/block/preallocate.c
+++ b/block/preallocate.c
@@ -78,6 +78,7 @@ typedef struct BDRVPreallocateState {
 
     /* Gives up the resize permission on children when parents don't need it */
     QEMUBH *drop_resize_bh;
+    bool    drop_resize_armed;
 } BDRVPreallocateState;
 
 static int preallocate_drop_resize(BlockDriverState *bs, Error **errp);
@@ -149,6 +150,7 @@ static int preallocate_open(BlockDriverState *bs, QDict *options, int flags,
      */
     s->file_end = s->zero_start = s->data_end = -EINVAL;
     s->drop_resize_bh = qemu_bh_new(preallocate_drop_resize_bh, bs);
+    s->drop_resize_armed = false;
 
     ret = bdrv_open_file_child(NULL, options, "file", bs, errp);
     if (ret < 0) {
@@ -200,7 +202,7 @@ static void preallocate_close(BlockDriverState *bs)
 {
     BDRVPreallocateState *s = bs->opaque;
 
-    qemu_bh_cancel(s->drop_resize_bh);
+    assert(!s->drop_resize_armed);
     qemu_bh_delete(s->drop_resize_bh);
 
     if (s->data_end >= 0) {
@@ -504,6 +506,8 @@ static int preallocate_drop_resize(BlockDriverState *bs, Error **errp)
     BDRVPreallocateState *s = bs->opaque;
     int ret;
 
+    s->drop_resize_armed = false;
+
     if (s->data_end < 0) {
         return 0;
     }
@@ -534,11 +538,15 @@ static int preallocate_drop_resize(BlockDriverState *bs, Error **errp)
 
 static void preallocate_drop_resize_bh(void *opaque)
 {
+    BlockDriverState *bs = opaque;
+
     /*
      * In case of errors, we'll simply keep the exclusive lock on the image
      * indefinitely.
      */
-    preallocate_drop_resize(opaque, NULL);
+    preallocate_drop_resize(bs, NULL);
+
+    bdrv_dec_in_flight(bs);
 }
 
 static void preallocate_set_perm(BlockDriverState *bs,
@@ -547,13 +555,13 @@ static void preallocate_set_perm(BlockDriverState *bs,
     BDRVPreallocateState *s = bs->opaque;
 
     if (can_write_resize(perm)) {
-        qemu_bh_cancel(s->drop_resize_bh);
         if (s->data_end < 0) {
             s->data_end = s->file_end = s->zero_start =
                 bs->file->bs->total_sectors * BDRV_SECTOR_SIZE;
         }
     } else {
-        qemu_bh_schedule(s->drop_resize_bh);
+        assert(!s->drop_resize_armed);
+        assert(s->data_end < 0);
     }
 }
 
@@ -592,6 +600,26 @@ static int preallocate_check_perm(BlockDriverState *bs, uint64_t perm,
     return 0;
 }
 
+static void preallocate_drain_begin(BlockDriverState *bs)
+{
+    BDRVPreallocateState *s = bs->opaque;
+
+    if (s->data_end < 0) {
+        return;
+    }
+    if (s->drop_resize_armed) {
+        return;
+    }
+    if (s->data_end == s->file_end) {
+        s->file_end = s->zero_start = s->data_end = -EINVAL;
+        return;
+    }
+
+    s->drop_resize_armed = true;
+    bdrv_inc_in_flight(bs);
+    qemu_bh_schedule(s->drop_resize_bh);
+}
+
 static BlockDriver bdrv_preallocate_filter = {
     .format_name = "preallocate",
     .instance_size = sizeof(BDRVPreallocateState),
@@ -600,6 +628,8 @@ static BlockDriver bdrv_preallocate_filter = {
     .bdrv_open            = preallocate_open,
     .bdrv_close           = preallocate_close,
 
+    .bdrv_drain_begin     = preallocate_drain_begin,
+
     .bdrv_reopen_prepare  = preallocate_reopen_prepare,
     .bdrv_reopen_commit   = preallocate_reopen_commit,
     .bdrv_reopen_abort    = preallocate_reopen_abort,
diff --git a/tests/qemu-iotests/298 b/tests/qemu-iotests/298
index 9e75ac6975..41f12685a7 100755
--- a/tests/qemu-iotests/298
+++ b/tests/qemu-iotests/298
@@ -94,8 +94,10 @@ class TestPreallocateFilter(TestPreallocateBase):
         self.assert_qmp(result, 'return', {})
         self.complete_and_wait()
 
-        # commit of new megabyte should trigger preallocation
-        self.check_big()
+        # commit of new megabyte should trigger preallocation, but drain
+        # will make file smaller
+        self.check_small()
+
 
     def test_reopen_opts(self):
         result = self.vm.qmp('blockdev-reopen', options=[{
-- 
2.43.5

Re: [PATCH 2/2] block/preallocate: fix image truncation logic

[Denis V. Lunev]

On 10/9/24 16:54, Andrey Drobyshev wrote:
> On 10/9/24 4:58 PM, Denis V. Lunev wrote:
>> Recent QEMU changes around preallocate_set_perm mandates that it is not
>> possible to poll on aio_context inside this function anymore. Thus
>> truncate operation has been moved inside bottom half. This bottom half
>> is scheduled from preallocate_set_perm() and that is all.
>>
>> This approach proven to be problematic in a lot of places once
>> additional operations are executed over preallocate filter in
>> production. The code validates that permissions have been really changed
>> just after the call to the set operation.
>>
>> All permissions operations or block driver graph changes are performed
>> inside the quiscent state in terms of the block layer. This means that
>> there are no in-flight packets which is guaranteed by the passing
>> through bdrv_drain() section.
>>
>> The idea is that we should effectively disable preallocate filter inside
>> bdrv_drain() and unblock permission changes. This section is definitely
>> not on the hot path and additional single truncate operation will not
>> hurt.
>>
>> Unfortunately bdrv_drain_begin() callback according to the documentation
>> also disallow waiting inside. Thus original approach with the bottom
>> half is not changed. bdrv_drain_begin() schedules the operation and in
>> order to ensure that it has been really executed before completion of
>> the section increments the amount of in-flight requests.
>>
>> Signed-off-by: Denis V. Lunev <den@openvz.org>
>> CC: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
>> CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
>> CC: Kevin Wolf <kwolf@redhat.com>
>> ---
>>   block/preallocate.c    | 38 ++++++++++++++++++++++++++++++++++----
>>   tests/qemu-iotests/298 |  6 ++++--
>>   2 files changed, 38 insertions(+), 6 deletions(-)
>>
>> diff --git a/block/preallocate.c b/block/preallocate.c
>> index 1cf854966c..d78ef0b045 100644
>> --- a/block/preallocate.c
>> +++ b/block/preallocate.c
>> @@ -78,6 +78,7 @@ typedef struct BDRVPreallocateState {
>>   
>>       /* Gives up the resize permission on children when parents don't need it */
>>       QEMUBH *drop_resize_bh;
>> +    bool    drop_resize_armed;
>>   } BDRVPreallocateState;
>>   
>>   static int preallocate_drop_resize(BlockDriverState *bs, Error **errp);
>> @@ -149,6 +150,7 @@ static int preallocate_open(BlockDriverState *bs, QDict *options, int flags,
>>        */
>>       s->file_end = s->zero_start = s->data_end = -EINVAL;
>>       s->drop_resize_bh = qemu_bh_new(preallocate_drop_resize_bh, bs);
>> +    s->drop_resize_armed = false;
>>   
>>       ret = bdrv_open_file_child(NULL, options, "file", bs, errp);
>>       if (ret < 0) {
>> @@ -200,7 +202,7 @@ static void preallocate_close(BlockDriverState *bs)
>>   {
>>       BDRVPreallocateState *s = bs->opaque;
>>   
>> -    qemu_bh_cancel(s->drop_resize_bh);
>> +    assert(!s->drop_resize_armed);
>>       qemu_bh_delete(s->drop_resize_bh);
>>   
>>       if (s->data_end >= 0) {
>> @@ -504,6 +506,8 @@ static int preallocate_drop_resize(BlockDriverState *bs, Error **errp)
>>       BDRVPreallocateState *s = bs->opaque;
>>       int ret;
>>   
>> +    s->drop_resize_armed = false;
>> +
>>       if (s->data_end < 0) {
>>           return 0;
>>       }
>> @@ -534,11 +538,15 @@ static int preallocate_drop_resize(BlockDriverState *bs, Error **errp)
>>   
>>   static void preallocate_drop_resize_bh(void *opaque)
>>   {
>> +    BlockDriverState *bs = opaque;
>> +
>>       /*
>>        * In case of errors, we'll simply keep the exclusive lock on the image
>>        * indefinitely.
>>        */
>> -    preallocate_drop_resize(opaque, NULL);
>> +    preallocate_drop_resize(bs, NULL);
>> +
>> +    bdrv_dec_in_flight(bs);
>>   }
>>   
>>   static void preallocate_set_perm(BlockDriverState *bs,
>> @@ -547,13 +555,13 @@ static void preallocate_set_perm(BlockDriverState *bs,
>>       BDRVPreallocateState *s = bs->opaque;
>>   
>>       if (can_write_resize(perm)) {
>> -        qemu_bh_cancel(s->drop_resize_bh);
>>           if (s->data_end < 0) {
>>               s->data_end = s->file_end = s->zero_start =
>>                   bs->file->bs->total_sectors * BDRV_SECTOR_SIZE;
>>           }
>>       } else {
>> -        qemu_bh_schedule(s->drop_resize_bh);
>> +        assert(!s->drop_resize_armed);
>> +        assert(s->data_end < 0);
>>       }
>>   }
>>   
>> @@ -592,6 +600,26 @@ static int preallocate_check_perm(BlockDriverState *bs, uint64_t perm,
>>       return 0;
>>   }
>>   
>> +static void preallocate_drain_begin(BlockDriverState *bs)
>> +{
>> +    BDRVPreallocateState *s = bs->opaque;
>> +
>> +    if (s->data_end < 0) {
>> +        return;
>> +    }
>> +    if (s->drop_resize_armed) {
>> +        return;
>> +    }
>> +    if (s->data_end == s->file_end) {
>> +        s->file_end = s->zero_start = s->data_end = -EINVAL;
>> +        return;
>> +    }
>> +
>> +    s->drop_resize_armed = true;
>> +    bdrv_inc_in_flight(bs);
>> +    qemu_bh_schedule(s->drop_resize_bh);
>> +}
>> +
>>   static BlockDriver bdrv_preallocate_filter = {
>>       .format_name = "preallocate",
>>       .instance_size = sizeof(BDRVPreallocateState),
>> @@ -600,6 +628,8 @@ static BlockDriver bdrv_preallocate_filter = {
>>       .bdrv_open            = preallocate_open,
>>       .bdrv_close           = preallocate_close,
>>   
>> +    .bdrv_drain_begin     = preallocate_drain_begin,
>> +
>>       .bdrv_reopen_prepare  = preallocate_reopen_prepare,
>>       .bdrv_reopen_commit   = preallocate_reopen_commit,
>>       .bdrv_reopen_abort    = preallocate_reopen_abort,
>> diff --git a/tests/qemu-iotests/298 b/tests/qemu-iotests/298
>> index 9e75ac6975..41f12685a7 100755
>> --- a/tests/qemu-iotests/298
>> +++ b/tests/qemu-iotests/298
>> @@ -94,8 +94,10 @@ class TestPreallocateFilter(TestPreallocateBase):
>>           self.assert_qmp(result, 'return', {})
>>           self.complete_and_wait()
>>   
>> -        # commit of new megabyte should trigger preallocation
>> -        self.check_big()
>> +        # commit of new megabyte should trigger preallocation, but drain
>> +        # will make file smaller
>> +        self.check_small()
>> +
>>   
>>       def test_reopen_opts(self):
>>           result = self.vm.qmp('blockdev-reopen', options=[{
> This patch doesn't seem to be applying cleanly to the current master branch
this is my fault. Thanks.

I will resend.

Den

Re: [PATCH 2/2] block/preallocate: fix image truncation logic

[Andrey Drobyshev]

On 10/9/24 4:58 PM, Denis V. Lunev wrote:
> Recent QEMU changes around preallocate_set_perm mandates that it is not
> possible to poll on aio_context inside this function anymore. Thus
> truncate operation has been moved inside bottom half. This bottom half
> is scheduled from preallocate_set_perm() and that is all.
> 
> This approach proven to be problematic in a lot of places once
> additional operations are executed over preallocate filter in
> production. The code validates that permissions have been really changed
> just after the call to the set operation.
> 
> All permissions operations or block driver graph changes are performed
> inside the quiscent state in terms of the block layer. This means that
> there are no in-flight packets which is guaranteed by the passing
> through bdrv_drain() section.
> 
> The idea is that we should effectively disable preallocate filter inside
> bdrv_drain() and unblock permission changes. This section is definitely
> not on the hot path and additional single truncate operation will not
> hurt.
> 
> Unfortunately bdrv_drain_begin() callback according to the documentation
> also disallow waiting inside. Thus original approach with the bottom
> half is not changed. bdrv_drain_begin() schedules the operation and in
> order to ensure that it has been really executed before completion of
> the section increments the amount of in-flight requests.
> 
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
> CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> CC: Kevin Wolf <kwolf@redhat.com>
> ---
>  block/preallocate.c    | 38 ++++++++++++++++++++++++++++++++++----
>  tests/qemu-iotests/298 |  6 ++++--
>  2 files changed, 38 insertions(+), 6 deletions(-)
> 
> diff --git a/block/preallocate.c b/block/preallocate.c
> index 1cf854966c..d78ef0b045 100644
> --- a/block/preallocate.c
> +++ b/block/preallocate.c
> @@ -78,6 +78,7 @@ typedef struct BDRVPreallocateState {
>  
>      /* Gives up the resize permission on children when parents don't need it */
>      QEMUBH *drop_resize_bh;
> +    bool    drop_resize_armed;
>  } BDRVPreallocateState;
>  
>  static int preallocate_drop_resize(BlockDriverState *bs, Error **errp);
> @@ -149,6 +150,7 @@ static int preallocate_open(BlockDriverState *bs, QDict *options, int flags,
>       */
>      s->file_end = s->zero_start = s->data_end = -EINVAL;
>      s->drop_resize_bh = qemu_bh_new(preallocate_drop_resize_bh, bs);
> +    s->drop_resize_armed = false;
>  
>      ret = bdrv_open_file_child(NULL, options, "file", bs, errp);
>      if (ret < 0) {
> @@ -200,7 +202,7 @@ static void preallocate_close(BlockDriverState *bs)
>  {
>      BDRVPreallocateState *s = bs->opaque;
>  
> -    qemu_bh_cancel(s->drop_resize_bh);
> +    assert(!s->drop_resize_armed);
>      qemu_bh_delete(s->drop_resize_bh);
>  
>      if (s->data_end >= 0) {
> @@ -504,6 +506,8 @@ static int preallocate_drop_resize(BlockDriverState *bs, Error **errp)
>      BDRVPreallocateState *s = bs->opaque;
>      int ret;
>  
> +    s->drop_resize_armed = false;
> +
>      if (s->data_end < 0) {
>          return 0;
>      }
> @@ -534,11 +538,15 @@ static int preallocate_drop_resize(BlockDriverState *bs, Error **errp)
>  
>  static void preallocate_drop_resize_bh(void *opaque)
>  {
> +    BlockDriverState *bs = opaque;
> +
>      /*
>       * In case of errors, we'll simply keep the exclusive lock on the image
>       * indefinitely.
>       */
> -    preallocate_drop_resize(opaque, NULL);
> +    preallocate_drop_resize(bs, NULL);
> +
> +    bdrv_dec_in_flight(bs);
>  }
>  
>  static void preallocate_set_perm(BlockDriverState *bs,
> @@ -547,13 +555,13 @@ static void preallocate_set_perm(BlockDriverState *bs,
>      BDRVPreallocateState *s = bs->opaque;
>  
>      if (can_write_resize(perm)) {
> -        qemu_bh_cancel(s->drop_resize_bh);
>          if (s->data_end < 0) {
>              s->data_end = s->file_end = s->zero_start =
>                  bs->file->bs->total_sectors * BDRV_SECTOR_SIZE;
>          }
>      } else {
> -        qemu_bh_schedule(s->drop_resize_bh);
> +        assert(!s->drop_resize_armed);
> +        assert(s->data_end < 0);
>      }
>  }
>  
> @@ -592,6 +600,26 @@ static int preallocate_check_perm(BlockDriverState *bs, uint64_t perm,
>      return 0;
>  }
>  
> +static void preallocate_drain_begin(BlockDriverState *bs)
> +{
> +    BDRVPreallocateState *s = bs->opaque;
> +
> +    if (s->data_end < 0) {
> +        return;
> +    }
> +    if (s->drop_resize_armed) {
> +        return;
> +    }
> +    if (s->data_end == s->file_end) {
> +        s->file_end = s->zero_start = s->data_end = -EINVAL;
> +        return;
> +    }
> +
> +    s->drop_resize_armed = true;
> +    bdrv_inc_in_flight(bs);
> +    qemu_bh_schedule(s->drop_resize_bh);
> +}
> +
>  static BlockDriver bdrv_preallocate_filter = {
>      .format_name = "preallocate",
>      .instance_size = sizeof(BDRVPreallocateState),
> @@ -600,6 +628,8 @@ static BlockDriver bdrv_preallocate_filter = {
>      .bdrv_open            = preallocate_open,
>      .bdrv_close           = preallocate_close,
>  
> +    .bdrv_drain_begin     = preallocate_drain_begin,
> +
>      .bdrv_reopen_prepare  = preallocate_reopen_prepare,
>      .bdrv_reopen_commit   = preallocate_reopen_commit,
>      .bdrv_reopen_abort    = preallocate_reopen_abort,
> diff --git a/tests/qemu-iotests/298 b/tests/qemu-iotests/298
> index 9e75ac6975..41f12685a7 100755
> --- a/tests/qemu-iotests/298
> +++ b/tests/qemu-iotests/298
> @@ -94,8 +94,10 @@ class TestPreallocateFilter(TestPreallocateBase):
>          self.assert_qmp(result, 'return', {})
>          self.complete_and_wait()
>  
> -        # commit of new megabyte should trigger preallocation
> -        self.check_big()
> +        # commit of new megabyte should trigger preallocation, but drain
> +        # will make file smaller
> +        self.check_small()
> +
>  
>      def test_reopen_opts(self):
>          result = self.vm.qmp('blockdev-reopen', options=[{

This patch doesn't seem to be applying cleanly to the current master branch

[PATCH v2 0/2] block/preallocate: fix image truncation logic

[Denis V. Lunev via]

Recent QEMU changes around preallocate_set_perm mandates that it is not
possible to poll on aio_context inside this function anymore. Thus
truncate operation has been moved inside bottom half. This bottom half
is scheduled from preallocate_set_perm() and that is all.

This approach proven to be problematic in a lot of places once
additional operations are executed over preallocate filter in
production. The code validates that permissions have been really changed
just after the call to the set operation.

All permissions operations or block driver graph changes are performed
inside the quiscent state in terms of the block layer. This means that
there are no in-flight packets which is guaranteed by the passing
through bdrv_drain() section.

The idea is that we should effectively disable preallocate filter inside
bdrv_drain() and unblock permission changes. This section is definitely
not on the hot path and additional single truncate operation will not
hurt.

Unfortunately bdrv_drain_begin() callback according to the documentation
also disallow waiting inside. Thus original approach with the bottom
half is not changed. bdrv_drain_begin() schedules the operation and in
order to ensure that it has been really executed before completion of
the section increments the amount of in-flight requests.

In addition to this we should disable lifting WRITE permission when
truncate() operation is not fully completed yet.

Changes from v1:
- rebased to the latest master

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
CC: Kevin Wolf <kwolf@redhat.com>

[PATCH 1/2] preallocate: do not allow to change BDS permission improperly

[Denis V. Lunev via]

RW permissions could not be lifted from the preallocation filter if
truncate operation has not been finished. In the other case this would
mean WRITE operation (image truncate) called after the return from
inactivate call. This is definitely a contract violation.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
CC: Kevin Wolf <kwolf@redhat.com>
---
 block/preallocate.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/block/preallocate.c b/block/preallocate.c
index d215bc5d6d..1016c511cb 100644
--- a/block/preallocate.c
+++ b/block/preallocate.c
@@ -594,6 +594,17 @@ static void preallocate_child_perm(BlockDriverState *bs, BdrvChild *c,
     }
 }
 
+static int preallocate_check_perm(BlockDriverState *bs, uint64_t perm,
+                                  uint64_t shared, Error **errp)
+{
+    BDRVPreallocateState *s = bs->opaque;
+    if (!can_write_resize(perm) && s->data_end != -EINVAL) {
+        error_setg_errno(errp, EPERM, "Write access is required for truncate");
+        return -EPERM;
+    }
+    return 0;
+}
+
 static BlockDriver bdrv_preallocate_filter = {
     .format_name = "preallocate",
     .instance_size = sizeof(BDRVPreallocateState),
@@ -615,6 +626,7 @@ static BlockDriver bdrv_preallocate_filter = {
 
     .bdrv_set_perm = preallocate_set_perm,
     .bdrv_child_perm = preallocate_child_perm,
+    .bdrv_check_perm = preallocate_check_perm,
 
     .is_filter = true,
 };
-- 
2.43.5

[PATCH 2/2] block/preallocate: fix image truncation logic

[Denis V. Lunev via]

Recent QEMU changes around preallocate_set_perm mandates that it is not
possible to poll on aio_context inside this function anymore. Thus
truncate operation has been moved inside bottom half. This bottom half
is scheduled from preallocate_set_perm() and that is all.

This approach proven to be problematic in a lot of places once
additional operations are executed over preallocate filter in
production. The code validates that permissions have been really changed
just after the call to the set operation.

All permissions operations or block driver graph changes are performed
inside the quiscent state in terms of the block layer. This means that
there are no in-flight packets which is guaranteed by the passing
through bdrv_drain() section.

The idea is that we should effectively disable preallocate filter inside
bdrv_drain() and unblock permission changes. This section is definitely
not on the hot path and additional single truncate operation will not
hurt.

Unfortunately bdrv_drain_begin() callback according to the documentation
also disallow waiting inside. Thus original approach with the bottom
half is not changed. bdrv_drain_begin() schedules the operation and in
order to ensure that it has been really executed before completion of
the section increments the amount of in-flight requests.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
CC: Kevin Wolf <kwolf@redhat.com>
---
 block/preallocate.c    | 42 ++++++++++++++++++++++++++++++++++++++----
 tests/qemu-iotests/298 |  6 ++++--
 2 files changed, 42 insertions(+), 6 deletions(-)

diff --git a/block/preallocate.c b/block/preallocate.c
index 1016c511cb..16a92a2e0d 100644
--- a/block/preallocate.c
+++ b/block/preallocate.c
@@ -78,6 +78,7 @@ typedef struct BDRVPreallocateState {
 
     /* Gives up the resize permission on children when parents don't need it */
     QEMUBH *drop_resize_bh;
+    bool    drop_resize_armed;
 } BDRVPreallocateState;
 
 static int preallocate_drop_resize(BlockDriverState *bs, Error **errp);
@@ -151,6 +152,7 @@ static int preallocate_open(BlockDriverState *bs, QDict *options, int flags,
      */
     s->file_end = s->zero_start = s->data_end = -EINVAL;
     s->drop_resize_bh = qemu_bh_new(preallocate_drop_resize_bh, bs);
+    s->drop_resize_armed = false;
 
     ret = bdrv_open_file_child(NULL, options, "file", bs, errp);
     if (ret < 0) {
@@ -208,7 +210,7 @@ static void preallocate_close(BlockDriverState *bs)
     GLOBAL_STATE_CODE();
     GRAPH_RDLOCK_GUARD_MAINLOOP();
 
-    qemu_bh_cancel(s->drop_resize_bh);
+    assert(!s->drop_resize_armed);
     qemu_bh_delete(s->drop_resize_bh);
 
     if (s->data_end >= 0) {
@@ -516,6 +518,8 @@ preallocate_drop_resize(BlockDriverState *bs, Error **errp)
     BDRVPreallocateState *s = bs->opaque;
     int ret;
 
+    s->drop_resize_armed = false;
+
     if (s->data_end < 0) {
         return 0;
     }
@@ -544,6 +548,12 @@ preallocate_drop_resize(BlockDriverState *bs, Error **errp)
 
 static void preallocate_drop_resize_bh(void *opaque)
 {
+    BlockDriverState *bs = opaque;
+
+     /*
+      * In case of errors, we'll simply keep the exclusive lock on the image
+      * indefinitely.
+      */
     GLOBAL_STATE_CODE();
     GRAPH_RDLOCK_GUARD_MAINLOOP();
 
@@ -551,7 +561,9 @@ static void preallocate_drop_resize_bh(void *opaque)
      * In case of errors, we'll simply keep the exclusive lock on the image
      * indefinitely.
      */
-    preallocate_drop_resize(opaque, NULL);
+    preallocate_drop_resize(bs, NULL);
+
+    bdrv_dec_in_flight(bs);
 }
 
 static void GRAPH_RDLOCK
@@ -560,13 +572,13 @@ preallocate_set_perm(BlockDriverState *bs, uint64_t perm, uint64_t shared)
     BDRVPreallocateState *s = bs->opaque;
 
     if (can_write_resize(perm)) {
-        qemu_bh_cancel(s->drop_resize_bh);
         if (s->data_end < 0) {
             s->data_end = s->file_end = s->zero_start =
                 bs->file->bs->total_sectors * BDRV_SECTOR_SIZE;
         }
     } else {
-        qemu_bh_schedule(s->drop_resize_bh);
+        assert(!s->drop_resize_armed);
+        assert(s->data_end < 0);
     }
 }
 
@@ -605,6 +617,26 @@ static int preallocate_check_perm(BlockDriverState *bs, uint64_t perm,
     return 0;
 }
 
+static void preallocate_drain_begin(BlockDriverState *bs)
+{
+    BDRVPreallocateState *s = bs->opaque;
+
+    if (s->data_end < 0) {
+        return;
+    }
+    if (s->drop_resize_armed) {
+        return;
+    }
+    if (s->data_end == s->file_end) {
+        s->file_end = s->zero_start = s->data_end = -EINVAL;
+        return;
+    }
+
+    s->drop_resize_armed = true;
+    bdrv_inc_in_flight(bs);
+    qemu_bh_schedule(s->drop_resize_bh);
+}
+
 static BlockDriver bdrv_preallocate_filter = {
     .format_name = "preallocate",
     .instance_size = sizeof(BDRVPreallocateState),
@@ -613,6 +645,8 @@ static BlockDriver bdrv_preallocate_filter = {
     .bdrv_open            = preallocate_open,
     .bdrv_close           = preallocate_close,
 
+    .bdrv_drain_begin     = preallocate_drain_begin,
+
     .bdrv_reopen_prepare  = preallocate_reopen_prepare,
     .bdrv_reopen_commit   = preallocate_reopen_commit,
     .bdrv_reopen_abort    = preallocate_reopen_abort,
diff --git a/tests/qemu-iotests/298 b/tests/qemu-iotests/298
index 09c9290711..fe03d29802 100755
--- a/tests/qemu-iotests/298
+++ b/tests/qemu-iotests/298
@@ -92,8 +92,10 @@ class TestPreallocateFilter(TestPreallocateBase):
         self.vm.cmd('block-commit', device='overlay')
         self.complete_and_wait()
 
-        # commit of new megabyte should trigger preallocation
-        self.check_big()
+        # commit of new megabyte should trigger preallocation, but drain
+        # will make file smaller
+        self.check_small()
+
 
     def test_reopen_opts(self):
         self.vm.cmd('blockdev-reopen', options=[{
-- 
2.43.5