[PATCH v2 0/9] drop unallocated_blocks_are_zero

[PATCH v2 0/9] drop unallocated_blocks_are_zero

[Vladimir Sementsov-Ogievskiy]

Hi all!

v2 (by Eric's review):

01: moved to the start of the series, add Eric's r-b
02: new
03-04: improve commit message
05: add Eric's r-b
06-08: improve commit message a bit, add Eric's r-b
09: typos and wording, rebase on 02


This is first step to block-status refactoring, and solves most simple
problem mentioned in my investigation of block-status described in
the thread "backing chain & block status & filters":
  https://lists.gnu.org/archive/html/qemu-devel/2020-04/msg04706.html


unallocated_blocks_are_zero doesn't simplify all the logic about
block-status, and happily it's not needed, as shown by the following
patches. So, let's get rid of it.

Vladimir Sementsov-Ogievskiy (9):
  qemu-img: convert: don't use unallocated_blocks_are_zero
  block: inline bdrv_unallocated_blocks_are_zero()
  block/vdi: return ZERO block-status when appropriate
  block/vpc: return ZERO block-status when appropriate
  block/crypto: drop unallocated_blocks_are_zero
  block/iscsi: drop unallocated_blocks_are_zero
  block/file-posix: drop unallocated_blocks_are_zero
  block/vhdx: drop unallocated_blocks_are_zero
  block: drop unallocated_blocks_are_zero

 include/block/block.h     |  6 ------
 include/block/block_int.h | 12 +++++++++++-
 block.c                   | 15 ---------------
 block/crypto.c            |  1 -
 block/file-posix.c        |  3 ---
 block/io.c                |  8 ++++----
 block/iscsi.c             |  1 -
 block/qcow2.c             |  1 -
 block/qed.c               |  1 -
 block/vdi.c               |  3 +--
 block/vhdx.c              |  3 ---
 block/vpc.c               |  3 +--
 qemu-img.c                |  4 +---
 13 files changed, 18 insertions(+), 43 deletions(-)

-- 
2.21.0

[PATCH v2 1/9] qemu-img: convert: don't use unallocated_blocks_are_zero

[Vladimir Sementsov-Ogievskiy]

qemu-img convert wants to distinguish ZERO which comes from short
backing files. unallocated_blocks_are_zero field of bdi is unrelated:
space after EOF is always considered to be zero anyway. So, just make
post_backing_zero true in case of short backing file.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
 qemu-img.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/qemu-img.c b/qemu-img.c
index 6a4327aaba..4fe751878b 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1632,7 +1632,6 @@ typedef struct ImgConvertState {
     BlockBackend *target;
     bool has_zero_init;
     bool compressed;
-    bool unallocated_blocks_are_zero;
     bool target_is_new;
     bool target_has_backing;
     int64_t target_backing_sectors; /* negative if unknown */
@@ -1677,7 +1676,7 @@ static int convert_iteration_sectors(ImgConvertState *s, int64_t sector_num)
 
     if (s->target_backing_sectors >= 0) {
         if (sector_num >= s->target_backing_sectors) {
-            post_backing_zero = s->unallocated_blocks_are_zero;
+            post_backing_zero = true;
         } else if (sector_num + n > s->target_backing_sectors) {
             /* Split requests around target_backing_sectors (because
              * starting from there, zeros are handled differently) */
@@ -2580,7 +2579,6 @@ static int img_convert(int argc, char **argv)
     } else {
         s.compressed = s.compressed || bdi.needs_compressed_writes;
         s.cluster_sectors = bdi.cluster_size / BDRV_SECTOR_SIZE;
-        s.unallocated_blocks_are_zero = bdi.unallocated_blocks_are_zero;
     }
 
     ret = convert_do_copy(&s);
-- 
2.21.0

[PATCH v2 2/9] block: inline bdrv_unallocated_blocks_are_zero()

[Vladimir Sementsov-Ogievskiy]

The function has the only user: bdrv_co_block_status(). Inline it to
simplify reviewing of the following patches, which will finally drop
unallocated_blocks_are_zero field too.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 include/block/block.h |  1 -
 block.c               | 15 ---------------
 block/io.c            | 11 ++++++++---
 3 files changed, 8 insertions(+), 19 deletions(-)

diff --git a/include/block/block.h b/include/block/block.h
index 8b62429aa4..931003a476 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -431,7 +431,6 @@ int bdrv_co_pdiscard(BdrvChild *child, int64_t offset, int64_t bytes);
 int bdrv_has_zero_init_1(BlockDriverState *bs);
 int bdrv_has_zero_init(BlockDriverState *bs);
 int bdrv_has_zero_init_truncate(BlockDriverState *bs);
-bool bdrv_unallocated_blocks_are_zero(BlockDriverState *bs);
 bool bdrv_can_write_zeroes_with_unmap(BlockDriverState *bs);
 int bdrv_block_status(BlockDriverState *bs, int64_t offset,
                       int64_t bytes, int64_t *pnum, int64_t *map,
diff --git a/block.c b/block.c
index cf5c19b1db..0283fdecbc 100644
--- a/block.c
+++ b/block.c
@@ -5305,21 +5305,6 @@ int bdrv_has_zero_init_truncate(BlockDriverState *bs)
     return 0;
 }
 
-bool bdrv_unallocated_blocks_are_zero(BlockDriverState *bs)
-{
-    BlockDriverInfo bdi;
-
-    if (bs->backing) {
-        return false;
-    }
-
-    if (bdrv_get_info(bs, &bdi) == 0) {
-        return bdi.unallocated_blocks_are_zero;
-    }
-
-    return false;
-}
-
 bool bdrv_can_write_zeroes_with_unmap(BlockDriverState *bs)
 {
     if (!(bs->open_flags & BDRV_O_UNMAP)) {
diff --git a/block/io.c b/block/io.c
index a4f9714230..00e7371d50 100644
--- a/block/io.c
+++ b/block/io.c
@@ -2386,15 +2386,20 @@ static int coroutine_fn bdrv_co_block_status(BlockDriverState *bs,
     if (ret & (BDRV_BLOCK_DATA | BDRV_BLOCK_ZERO)) {
         ret |= BDRV_BLOCK_ALLOCATED;
     } else if (want_zero) {
-        if (bdrv_unallocated_blocks_are_zero(bs)) {
-            ret |= BDRV_BLOCK_ZERO;
-        } else if (bs->backing) {
+        if (bs->backing) {
             BlockDriverState *bs2 = bs->backing->bs;
             int64_t size2 = bdrv_getlength(bs2);
 
             if (size2 >= 0 && offset >= size2) {
                 ret |= BDRV_BLOCK_ZERO;
             }
+        } else {
+            BlockDriverInfo bdi;
+            int ret2 = bdrv_get_info(bs, &bdi);
+
+            if (ret2 == 0 && bdi.unallocated_blocks_are_zero) {
+                ret |= BDRV_BLOCK_ZERO;
+            }
         }
     }
 
-- 
2.21.0

[PATCH v2 3/9] block/vdi: return ZERO block-status when appropriate

[Vladimir Sementsov-Ogievskiy]

In case of !VDI_IS_ALLOCATED[], we do zero out the corresponding chunk
of qiov. So, this should be reported as ZERO.

Note that this changes visible output of "qemu-img map --output=json"
and "qemu-io -c map" commands. For qemu-img map, the change is obvious:
we just mark as zero what is really zero. For qemu-io it's less
obvious: what was unallocated now is allocated.

There is an inconsistency in understanding of unallocated regions in
Qemu: backing-supporting format-drivers return 0 block-status to report
go-to-backing logic for this area. Some protocol-drivers (iscsi) return
0 to report fs-unallocated-non-zero status (i.e., don't occupy space on
disk, read result is undefined).

BDRV_BLOCK_ALLOCATED is defined as something more close to
go-to-backing logic. Still it is calculated as ZERO | DATA, so 0 from
iscsi is treated as unallocated. It doesn't influence backing-chain
behavior, as iscsi can't have backing file. But it does influence
"qemu-io -c map".

We should solve this inconsistency at some future point. Now, let's
just make backing-not-supporting format drivers (vdi at this patch and
vpc with the following) to behave more like backing-supporting drivers
and not report 0 block-status. More over, returning ZERO status is
absolutely valid thing, and again, corresponds to how the other
format-drivers (backing-supporting) work.

After block-status update, it never reports 0, so setting
unallocated_blocks_are_zero doesn't make sense (as the only user of it
is bdrv_co_block_status and it checks unallocated_blocks_are_zero only
for unallocated areas). Drop it.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/vdi.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/block/vdi.c b/block/vdi.c
index 0c7835ae70..83471528d2 100644
--- a/block/vdi.c
+++ b/block/vdi.c
@@ -334,7 +334,6 @@ static int vdi_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
     logout("\n");
     bdi->cluster_size = s->block_size;
     bdi->vm_state_offset = 0;
-    bdi->unallocated_blocks_are_zero = true;
     return 0;
 }
 
@@ -536,7 +535,7 @@ static int coroutine_fn vdi_co_block_status(BlockDriverState *bs,
     *pnum = MIN(s->block_size - index_in_block, bytes);
     result = VDI_IS_ALLOCATED(bmap_entry);
     if (!result) {
-        return 0;
+        return BDRV_BLOCK_ZERO;
     }
 
     *map = s->header.offset_data + (uint64_t)bmap_entry * s->block_size +
-- 
2.21.0

[PATCH v2 4/9] block/vpc: return ZERO block-status when appropriate

[Vladimir Sementsov-Ogievskiy]

In case when get_image_offset() returns -1, we do zero out the
corresponding chunk of qiov. So, this should be reported as ZERO.

Note that this changes visible output of "qemu-img map --output=json"
and "qemu-io -c map" commands. For qemu-img map, the change is obvious:
we just mark as zero what is really zero. For qemu-io it's less
obvious: what was unallocated now is allocated.

There is an inconsistency in understanding of unallocated regions in
Qemu: backing-supporting format-drivers return 0 block-status to report
go-to-backing logic for this area. Some protocol-drivers (iscsi) return
0 to report fs-unallocated-non-zero status (i.e., don't occupy space on
disk, read result is undefined).

BDRV_BLOCK_ALLOCATED is defined as something more close to
go-to-backing logic. Still it is calculated as ZERO | DATA, so 0 from
iscsi is treated as unallocated. It doesn't influence backing-chain
behavior, as iscsi can't have backing file. But it does influence
"qemu-io -c map".

We should solve this inconsistency at some future point. Now, let's
just make backing-not-supporting format drivers (vdi in the previous
patch and vpc now) to behave more like backing-supporting drivers
and not report 0 block-status. More over, returning ZERO status is
absolutely valid thing, and again, corresponds to how the other
format-drivers (backing-supporting) work.

After block-status update, it never reports 0, so setting
unallocated_blocks_are_zero doesn't make sense (as the only user of it
is bdrv_co_block_status and it checks unallocated_blocks_are_zero only
for unallocated areas). Drop it.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/vpc.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/block/vpc.c b/block/vpc.c
index 2d1eade146..555f9d8587 100644
--- a/block/vpc.c
+++ b/block/vpc.c
@@ -606,7 +606,6 @@ static int vpc_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
         bdi->cluster_size = s->block_size;
     }
 
-    bdi->unallocated_blocks_are_zero = true;
     return 0;
 }
 
@@ -745,7 +744,7 @@ static int coroutine_fn vpc_co_block_status(BlockDriverState *bs,
     image_offset = get_image_offset(bs, offset, false, NULL);
     allocated = (image_offset != -1);
     *pnum = 0;
-    ret = 0;
+    ret = BDRV_BLOCK_ZERO;
 
     do {
         /* All sectors in a block are contiguous (without using the bitmap) */
-- 
2.21.0

[PATCH v2 5/9] block/crypto: drop unallocated_blocks_are_zero

[Vladimir Sementsov-Ogievskiy]

It's false by default, no needs to set it. We are going to drop this
variable at all, so drop it now here, it doesn't hurt.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
 block/crypto.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/block/crypto.c b/block/crypto.c
index e02f343590..7685e61844 100644
--- a/block/crypto.c
+++ b/block/crypto.c
@@ -694,7 +694,6 @@ static int block_crypto_get_info_luks(BlockDriverState *bs,
         return ret;
     }
 
-    bdi->unallocated_blocks_are_zero = false;
     bdi->cluster_size = subbdi.cluster_size;
 
     return 0;
-- 
2.21.0

[PATCH v2 6/9] block/iscsi: drop unallocated_blocks_are_zero

[Vladimir Sementsov-Ogievskiy]

We set bdi->unallocated_blocks_are_zero = iscsilun->lbprz, but
iscsi_co_block_status doesn't return 0 in case of iscsilun->lbprz, it
returns ZERO when appropriate. So actually unallocated_blocks_are_zero
is useless (it doesn't affect the only user of the field:
bdrv_co_block_status()). Drop it now.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
 block/iscsi.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/block/iscsi.c b/block/iscsi.c
index a8b76979d8..767e3e75fd 100644
--- a/block/iscsi.c
+++ b/block/iscsi.c
@@ -2163,7 +2163,6 @@ static int coroutine_fn iscsi_co_truncate(BlockDriverState *bs, int64_t offset,
 static int iscsi_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
 {
     IscsiLun *iscsilun = bs->opaque;
-    bdi->unallocated_blocks_are_zero = iscsilun->lbprz;
     bdi->cluster_size = iscsilun->cluster_size;
     return 0;
 }
-- 
2.21.0

[PATCH v2 7/9] block/file-posix: drop unallocated_blocks_are_zero

[Vladimir Sementsov-Ogievskiy]

raw_co_block_status() in block/file-posix.c never returns 0, so
unallocated_blocks_are_zero is useless (it doesn't affect the only user
of the field: bdrv_co_block_status()). Drop it.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
 block/file-posix.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index 05e094be29..5c01735108 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -2878,9 +2878,6 @@ static int coroutine_fn raw_co_pwrite_zeroes(
 
 static int raw_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
 {
-    BDRVRawState *s = bs->opaque;
-
-    bdi->unallocated_blocks_are_zero = s->discard_zeroes;
     return 0;
 }
 
-- 
2.21.0

[PATCH v2 8/9] block/vhdx: drop unallocated_blocks_are_zero

[Vladimir Sementsov-Ogievskiy]

vhdx doesn't have .bdrv_co_block_status handler, so DATA|ALLOCATED is
always assumed for it in bdrv_co_block_status().
unallocated_blocks_are_zero is useless (it doesn't affect the only user
of the field: bdrv_co_block_status()), drop it.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
 block/vhdx.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/block/vhdx.c b/block/vhdx.c
index aedd782604..45963a3166 100644
--- a/block/vhdx.c
+++ b/block/vhdx.c
@@ -1164,9 +1164,6 @@ static int vhdx_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
 
     bdi->cluster_size = s->block_size;
 
-    bdi->unallocated_blocks_are_zero =
-        (s->params.data_bits & VHDX_PARAMS_HAS_PARENT) == 0;
-
     return 0;
 }
 
-- 
2.21.0

[PATCH v2 9/9] block: drop unallocated_blocks_are_zero

[Vladimir Sementsov-Ogievskiy]

Currently this field only set by qed and qcow2. But in fact, all
backing-supporting formats (parallels, qcow, qcow2, qed, vmdk) share
these semantics: on unallocated blocks, if there is no backing file they
just memset the buffer with zeroes.

So, document this behavior for .supports_backing and drop
.unallocated_blocks_are_zero

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 include/block/block.h     |  5 -----
 include/block/block_int.h | 12 +++++++++++-
 block/io.c                |  9 ++-------
 block/qcow2.c             |  1 -
 block/qed.c               |  1 -
 5 files changed, 13 insertions(+), 15 deletions(-)

diff --git a/include/block/block.h b/include/block/block.h
index 931003a476..db1cb503ec 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -21,11 +21,6 @@ typedef struct BlockDriverInfo {
     /* offset at which the VM state can be saved (0 if not possible) */
     int64_t vm_state_offset;
     bool is_dirty;
-    /*
-     * True if unallocated blocks read back as zeroes. This is equivalent
-     * to the LBPRZ flag in the SCSI logical block provisioning page.
-     */
-    bool unallocated_blocks_are_zero;
     /*
      * True if this block driver only supports compressed writes
      */
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 92335f33c7..8fac6c3ce2 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -115,7 +115,17 @@ struct BlockDriver {
      */
     bool bdrv_needs_filename;
 
-    /* Set if a driver can support backing files */
+    /*
+     * Set if a driver can support backing files. This also implies the
+     * following semantics:
+     *
+     *  - Return status 0 of .bdrv_co_block_status means that corresponding
+     *    blocks are not allocated in this layer of backing-chain
+     *  - For such (unallocated) blocks, read will:
+     *    - fill buffer with zeros if there is no backing file
+     *    - read from the backing file otherwise, where the block layer
+     *      takes care of reading zeros beyond EOF if backing file is short
+     */
     bool supports_backing;
 
     /* For handling image reopen for split or non-split files */
diff --git a/block/io.c b/block/io.c
index 00e7371d50..484adec5a1 100644
--- a/block/io.c
+++ b/block/io.c
@@ -2385,7 +2385,7 @@ static int coroutine_fn bdrv_co_block_status(BlockDriverState *bs,
 
     if (ret & (BDRV_BLOCK_DATA | BDRV_BLOCK_ZERO)) {
         ret |= BDRV_BLOCK_ALLOCATED;
-    } else if (want_zero) {
+    } else if (want_zero && bs->drv->supports_backing) {
         if (bs->backing) {
             BlockDriverState *bs2 = bs->backing->bs;
             int64_t size2 = bdrv_getlength(bs2);
@@ -2394,12 +2394,7 @@ static int coroutine_fn bdrv_co_block_status(BlockDriverState *bs,
                 ret |= BDRV_BLOCK_ZERO;
             }
         } else {
-            BlockDriverInfo bdi;
-            int ret2 = bdrv_get_info(bs, &bdi);
-
-            if (ret2 == 0 && bdi.unallocated_blocks_are_zero) {
-                ret |= BDRV_BLOCK_ZERO;
-            }
+            ret |= BDRV_BLOCK_ZERO;
         }
     }
 
diff --git a/block/qcow2.c b/block/qcow2.c
index 2ba0b17c39..dc3c0aac2b 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -4858,7 +4858,6 @@ err:
 static int qcow2_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
 {
     BDRVQcow2State *s = bs->opaque;
-    bdi->unallocated_blocks_are_zero = true;
     bdi->cluster_size = s->cluster_size;
     bdi->vm_state_offset = qcow2_vm_state_offset(s);
     return 0;
diff --git a/block/qed.c b/block/qed.c
index b0fdb8f565..fb7833dc8b 100644
--- a/block/qed.c
+++ b/block/qed.c
@@ -1514,7 +1514,6 @@ static int bdrv_qed_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
     memset(bdi, 0, sizeof(*bdi));
     bdi->cluster_size = s->header.cluster_size;
     bdi->is_dirty = s->header.features & QED_F_NEED_CHECK;
-    bdi->unallocated_blocks_are_zero = true;
     return 0;
 }
 
-- 
2.21.0

Re: [PATCH v2 2/9] block: inline bdrv_unallocated_blocks_are_zero()

[Eric Blake]

On 5/7/20 3:47 AM, Vladimir Sementsov-Ogievskiy wrote:
> The function has the only user: bdrv_co_block_status(). Inline it to

s/the only/only one/

> simplify reviewing of the following patches, which will finally drop
> unallocated_blocks_are_zero field too.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   include/block/block.h |  1 -
>   block.c               | 15 ---------------
>   block/io.c            | 11 ++++++++---
>   3 files changed, 8 insertions(+), 19 deletions(-)
> 

> +++ b/block/io.c
> @@ -2386,15 +2386,20 @@ static int coroutine_fn bdrv_co_block_status(BlockDriverState *bs,
>       if (ret & (BDRV_BLOCK_DATA | BDRV_BLOCK_ZERO)) {
>           ret |= BDRV_BLOCK_ALLOCATED;
>       } else if (want_zero) {
> -        if (bdrv_unallocated_blocks_are_zero(bs)) {
> -            ret |= BDRV_BLOCK_ZERO;
> -        } else if (bs->backing) {
> +        if (bs->backing) {
>               BlockDriverState *bs2 = bs->backing->bs;
>               int64_t size2 = bdrv_getlength(bs2);
>   
>               if (size2 >= 0 && offset >= size2) {
>                   ret |= BDRV_BLOCK_ZERO;
>               }
> +        } else {
> +            BlockDriverInfo bdi;
> +            int ret2 = bdrv_get_info(bs, &bdi);
> +
> +            if (ret2 == 0 && bdi.unallocated_blocks_are_zero) {

Could perhaps condense to:

else {
     BlockDriverInfo bdi;

     if (bdrv_get_info(bs, &bd) == 0 &&
         bdi.unallocated_blocks_are_zero) {

but that's cosmetic.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

Re: [PATCH v2 3/9] block/vdi: return ZERO block-status when appropriate

[Eric Blake]

On 5/7/20 3:47 AM, Vladimir Sementsov-Ogievskiy wrote:
> In case of !VDI_IS_ALLOCATED[], we do zero out the corresponding chunk
> of qiov. So, this should be reported as ZERO.
> 
> Note that this changes visible output of "qemu-img map --output=json"
> and "qemu-io -c map" commands. For qemu-img map, the change is obvious:
> we just mark as zero what is really zero. For qemu-io it's less
> obvious: what was unallocated now is allocated.
> 
> There is an inconsistency in understanding of unallocated regions in
> Qemu: backing-supporting format-drivers return 0 block-status to report
> go-to-backing logic for this area. Some protocol-drivers (iscsi) return
> 0 to report fs-unallocated-non-zero status (i.e., don't occupy space on
> disk, read result is undefined).
> 
> BDRV_BLOCK_ALLOCATED is defined as something more close to
> go-to-backing logic. Still it is calculated as ZERO | DATA, so 0 from
> iscsi is treated as unallocated. It doesn't influence backing-chain
> behavior, as iscsi can't have backing file. But it does influence
> "qemu-io -c map".
> 
> We should solve this inconsistency at some future point. Now, let's
> just make backing-not-supporting format drivers (vdi at this patch and
> vpc with the following) to behave more like backing-supporting drivers
> and not report 0 block-status. More over, returning ZERO status is
> absolutely valid thing, and again, corresponds to how the other
> format-drivers (backing-supporting) work.
> 
> After block-status update, it never reports 0, so setting
> unallocated_blocks_are_zero doesn't make sense (as the only user of it
> is bdrv_co_block_status and it checks unallocated_blocks_are_zero only
> for unallocated areas). Drop it.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   block/vdi.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 

Yes, much better commit message, showing why the code change is correct.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

Re: [PATCH v2 4/9] block/vpc: return ZERO block-status when appropriate

[Eric Blake]

On 5/7/20 3:47 AM, Vladimir Sementsov-Ogievskiy wrote:
> In case when get_image_offset() returns -1, we do zero out the
> corresponding chunk of qiov. So, this should be reported as ZERO.
> 
> Note that this changes visible output of "qemu-img map --output=json"
> and "qemu-io -c map" commands. For qemu-img map, the change is obvious:
> we just mark as zero what is really zero. For qemu-io it's less
> obvious: what was unallocated now is allocated.
> 
> There is an inconsistency in understanding of unallocated regions in
> Qemu: backing-supporting format-drivers return 0 block-status to report
> go-to-backing logic for this area. Some protocol-drivers (iscsi) return
> 0 to report fs-unallocated-non-zero status (i.e., don't occupy space on
> disk, read result is undefined).
> 
> BDRV_BLOCK_ALLOCATED is defined as something more close to
> go-to-backing logic. Still it is calculated as ZERO | DATA, so 0 from
> iscsi is treated as unallocated. It doesn't influence backing-chain
> behavior, as iscsi can't have backing file. But it does influence
> "qemu-io -c map".
> 
> We should solve this inconsistency at some future point. Now, let's
> just make backing-not-supporting format drivers (vdi in the previous
> patch and vpc now) to behave more like backing-supporting drivers
> and not report 0 block-status. More over, returning ZERO status is
> absolutely valid thing, and again, corresponds to how the other
> format-drivers (backing-supporting) work.
> 
> After block-status update, it never reports 0, so setting
> unallocated_blocks_are_zero doesn't make sense (as the only user of it
> is bdrv_co_block_status and it checks unallocated_blocks_are_zero only
> for unallocated areas). Drop it.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   block/vpc.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

Re: [PATCH v2 9/9] block: drop unallocated_blocks_are_zero

[Eric Blake]

On 5/7/20 3:48 AM, Vladimir Sementsov-Ogievskiy wrote:
> Currently this field only set by qed and qcow2. But in fact, all
> backing-supporting formats (parallels, qcow, qcow2, qed, vmdk) share
> these semantics: on unallocated blocks, if there is no backing file they
> just memset the buffer with zeroes.
> 
> So, document this behavior for .supports_backing and drop
> .unallocated_blocks_are_zero
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   include/block/block.h     |  5 -----
>   include/block/block_int.h | 12 +++++++++++-
>   block/io.c                |  9 ++-------
>   block/qcow2.c             |  1 -
>   block/qed.c               |  1 -
>   5 files changed, 13 insertions(+), 15 deletions(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

[PATCH v2 10/9] qed: Simplify backing reads

[Eric Blake]

The other four drivers that support backing files (qcow, qcow2,
parallels, vmdk) all rely on the block layer to populate zeroes when
reading beyond EOF of a short backing file.  We can simplify the qed
code by doing likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>
---

I noticed this during my audit that v1 of Vladimir's series was correct.

No change in iotests results (test 274 is currently failing for qed,
but for other reasons:
+Traceback (most recent call last):
+  File "274", line 24, in <module>
+    iotests.verify_image_format(supported_fmts=['qcow2'])
+AttributeError: module 'iotests' has no attribute 'verify_image_format'
)

 block/qed.h |  1 -
 block/qed.c | 64 +++++------------------------------------------------
 2 files changed, 6 insertions(+), 59 deletions(-)

diff --git a/block/qed.h b/block/qed.h
index 42c115d8220c..3d12bf78d412 100644
--- a/block/qed.h
+++ b/block/qed.h
@@ -140,7 +140,6 @@ typedef struct QEDAIOCB {

     /* Current cluster scatter-gather list */
     QEMUIOVector cur_qiov;
-    QEMUIOVector *backing_qiov;
     uint64_t cur_pos;               /* position on block device, in bytes */
     uint64_t cur_cluster;           /* cluster offset in image file */
     unsigned int cur_nclusters;     /* number of clusters being accessed */
diff --git a/block/qed.c b/block/qed.c
index 927382995a0c..bea4b9f6cc97 100644
--- a/block/qed.c
+++ b/block/qed.c
@@ -849,56 +849,18 @@ static BDRVQEDState *acb_to_s(QEDAIOCB *acb)
  * @s:              QED state
  * @pos:            Byte position in device
  * @qiov:           Destination I/O vector
- * @backing_qiov:   Possibly shortened copy of qiov, to be allocated here
- * @cb:             Completion function
- * @opaque:         User data for completion function
  *
  * This function reads qiov->size bytes starting at pos from the backing file.
  * If there is no backing file then zeroes are read.
  */
 static int coroutine_fn qed_read_backing_file(BDRVQEDState *s, uint64_t pos,
-                                              QEMUIOVector *qiov,
-                                              QEMUIOVector **backing_qiov)
+                                              QEMUIOVector *qiov)
 {
-    uint64_t backing_length = 0;
-    size_t size;
-    int ret;
-
-    /* If there is a backing file, get its length.  Treat the absence of a
-     * backing file like a zero length backing file.
-     */
     if (s->bs->backing) {
-        int64_t l = bdrv_getlength(s->bs->backing->bs);
-        if (l < 0) {
-            return l;
-        }
-        backing_length = l;
-    }
-
-    /* Zero all sectors if reading beyond the end of the backing file */
-    if (pos >= backing_length ||
-        pos + qiov->size > backing_length) {
-        qemu_iovec_memset(qiov, 0, 0, qiov->size);
-    }
-
-    /* Complete now if there are no backing file sectors to read */
-    if (pos >= backing_length) {
-        return 0;
-    }
-
-    /* If the read straddles the end of the backing file, shorten it */
-    size = MIN((uint64_t)backing_length - pos, qiov->size);
-
-    assert(*backing_qiov == NULL);
-    *backing_qiov = g_new(QEMUIOVector, 1);
-    qemu_iovec_init(*backing_qiov, qiov->niov);
-    qemu_iovec_concat(*backing_qiov, qiov, 0, size);
-
-    BLKDBG_EVENT(s->bs->file, BLKDBG_READ_BACKING_AIO);
-    ret = bdrv_co_preadv(s->bs->backing, pos, size, *backing_qiov, 0);
-    if (ret < 0) {
-        return ret;
+        BLKDBG_EVENT(s->bs->file, BLKDBG_READ_BACKING_AIO);
+        return bdrv_co_preadv(s->bs->backing, pos, qiov->size, qiov, 0);
     }
+    qemu_iovec_memset(qiov, 0, 0, qiov->size);
     return 0;
 }

@@ -915,7 +877,6 @@ static int coroutine_fn qed_copy_from_backing_file(BDRVQEDState *s,
                                                    uint64_t offset)
 {
     QEMUIOVector qiov;
-    QEMUIOVector *backing_qiov = NULL;
     int ret;

     /* Skip copy entirely if there is no work to do */
@@ -925,13 +886,7 @@ static int coroutine_fn qed_copy_from_backing_file(BDRVQEDState *s,

     qemu_iovec_init_buf(&qiov, qemu_blockalign(s->bs, len), len);

-    ret = qed_read_backing_file(s, pos, &qiov, &backing_qiov);
-
-    if (backing_qiov) {
-        qemu_iovec_destroy(backing_qiov);
-        g_free(backing_qiov);
-        backing_qiov = NULL;
-    }
+    ret = qed_read_backing_file(s, pos, &qiov);

     if (ret) {
         goto out;
@@ -1339,8 +1294,7 @@ static int coroutine_fn qed_aio_read_data(void *opaque, int ret,
         qemu_iovec_memset(&acb->cur_qiov, 0, 0, acb->cur_qiov.size);
         r = 0;
     } else if (ret != QED_CLUSTER_FOUND) {
-        r = qed_read_backing_file(s, acb->cur_pos, &acb->cur_qiov,
-                                  &acb->backing_qiov);
+        r = qed_read_backing_file(s, acb->cur_pos, &acb->cur_qiov);
     } else {
         BLKDBG_EVENT(bs->file, BLKDBG_READ_AIO);
         r = bdrv_co_preadv(bs->file, offset, acb->cur_qiov.size,
@@ -1365,12 +1319,6 @@ static int coroutine_fn qed_aio_next_io(QEDAIOCB *acb)
     while (1) {
         trace_qed_aio_next_io(s, acb, 0, acb->cur_pos + acb->cur_qiov.size);

-        if (acb->backing_qiov) {
-            qemu_iovec_destroy(acb->backing_qiov);
-            g_free(acb->backing_qiov);
-            acb->backing_qiov = NULL;
-        }
-
         acb->qiov_offset += acb->cur_qiov.size;
         acb->cur_pos += acb->cur_qiov.size;
         qemu_iovec_reset(&acb->cur_qiov);
-- 
2.26.2

Re: [PATCH v2 10/9] qed: Simplify backing reads

[Eric Blake]

On 5/7/20 9:45 AM, Eric Blake wrote:
> The other four drivers that support backing files (qcow, qcow2,
> parallels, vmdk) all rely on the block layer to populate zeroes when
> reading beyond EOF of a short backing file.  We can simplify the qed
> code by doing likewise.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
> 
> I noticed this during my audit that v1 of Vladimir's series was correct.
> 
> No change in iotests results (test 274 is currently failing for qed,
> but for other reasons:
> +Traceback (most recent call last):
> +  File "274", line 24, in <module>
> +    iotests.verify_image_format(supported_fmts=['qcow2'])
> +AttributeError: module 'iotests' has no attribute 'verify_image_format'
> )

That iotest failure was due to a stale branch on my end; after updating 
to latest git master plus Kevin's latest 'block' branch, 274 is now 
skipped on qed.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

Re: [PATCH v2 10/9] qed: Simplify backing reads

[Vladimir Sementsov-Ogievskiy]

07.05.2020 17:45, Eric Blake wrote:
> The other four drivers that support backing files (qcow, qcow2,
> parallels, vmdk) all rely on the block layer to populate zeroes when
> reading beyond EOF of a short backing file.  We can simplify the qed
> code by doing likewise.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

-- 
Best regards,
Vladimir

Re: [PATCH v2 0/9] drop unallocated_blocks_are_zero

[Vladimir Sementsov-Ogievskiy]

Ping

The whole series reviewed by Eric, with only one grammar fix needed in 02 commit message (and possible drop of ret2, up to maintainer).

07.05.2020 11:47, Vladimir Sementsov-Ogievskiy wrote:
> Hi all!
> 
> v2 (by Eric's review):
> 
> 01: moved to the start of the series, add Eric's r-b
> 02: new
> 03-04: improve commit message
> 05: add Eric's r-b
> 06-08: improve commit message a bit, add Eric's r-b
> 09: typos and wording, rebase on 02
> 
> 
> This is first step to block-status refactoring, and solves most simple
> problem mentioned in my investigation of block-status described in
> the thread "backing chain & block status & filters":
>    https://lists.gnu.org/archive/html/qemu-devel/2020-04/msg04706.html
> 
> 
> unallocated_blocks_are_zero doesn't simplify all the logic about
> block-status, and happily it's not needed, as shown by the following
> patches. So, let's get rid of it.
> 
> Vladimir Sementsov-Ogievskiy (9):
>    qemu-img: convert: don't use unallocated_blocks_are_zero
>    block: inline bdrv_unallocated_blocks_are_zero()
>    block/vdi: return ZERO block-status when appropriate
>    block/vpc: return ZERO block-status when appropriate
>    block/crypto: drop unallocated_blocks_are_zero
>    block/iscsi: drop unallocated_blocks_are_zero
>    block/file-posix: drop unallocated_blocks_are_zero
>    block/vhdx: drop unallocated_blocks_are_zero
>    block: drop unallocated_blocks_are_zero
> 
>   include/block/block.h     |  6 ------
>   include/block/block_int.h | 12 +++++++++++-
>   block.c                   | 15 ---------------
>   block/crypto.c            |  1 -
>   block/file-posix.c        |  3 ---
>   block/io.c                |  8 ++++----
>   block/iscsi.c             |  1 -
>   block/qcow2.c             |  1 -
>   block/qed.c               |  1 -
>   block/vdi.c               |  3 +--
>   block/vhdx.c              |  3 ---
>   block/vpc.c               |  3 +--
>   qemu-img.c                |  4 +---
>   13 files changed, 18 insertions(+), 43 deletions(-)
> 


-- 
Best regards,
Vladimir

Re: [PATCH v2 2/9] block: inline bdrv_unallocated_blocks_are_zero()

[Vladimir Sementsov-Ogievskiy]

07.05.2020 17:08, Eric Blake wrote:
> On 5/7/20 3:47 AM, Vladimir Sementsov-Ogievskiy wrote:
>> The function has the only user: bdrv_co_block_status(). Inline it to
> 
> s/the only/only one/
> 
>> simplify reviewing of the following patches, which will finally drop
>> unallocated_blocks_are_zero field too.
>>
>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>> ---
>>   include/block/block.h |  1 -
>>   block.c               | 15 ---------------
>>   block/io.c            | 11 ++++++++---
>>   3 files changed, 8 insertions(+), 19 deletions(-)
>>
> 
>> +++ b/block/io.c
>> @@ -2386,15 +2386,20 @@ static int coroutine_fn bdrv_co_block_status(BlockDriverState *bs,
>>       if (ret & (BDRV_BLOCK_DATA | BDRV_BLOCK_ZERO)) {
>>           ret |= BDRV_BLOCK_ALLOCATED;
>>       } else if (want_zero) {
>> -        if (bdrv_unallocated_blocks_are_zero(bs)) {
>> -            ret |= BDRV_BLOCK_ZERO;
>> -        } else if (bs->backing) {
>> +        if (bs->backing) {
>>               BlockDriverState *bs2 = bs->backing->bs;
>>               int64_t size2 = bdrv_getlength(bs2);
>>               if (size2 >= 0 && offset >= size2) {
>>                   ret |= BDRV_BLOCK_ZERO;
>>               }
>> +        } else {
>> +            BlockDriverInfo bdi;
>> +            int ret2 = bdrv_get_info(bs, &bdi);
>> +
>> +            if (ret2 == 0 && bdi.unallocated_blocks_are_zero) {
> 
> Could perhaps condense to:
> 
> else {
>      BlockDriverInfo bdi;
> 
>      if (bdrv_get_info(bs, &bd) == 0 &&
>          bdi.unallocated_blocks_are_zero) {
> 
> but that's cosmetic.

I'll keep it as is, as it is to be removed in 09 anyway.

> 
> Reviewed-by: Eric Blake <eblake@redhat.com>
> 


-- 
Best regards,
Vladimir