From 4290173219e15065e9a7c2e95774ac979b5fd869 Mon Sep 17 00:00:00 2001
From: Kevin Wolf <kwolf@redhat.com>
Date: Mon, 8 Jun 2020 15:01:40 +0100
Subject: [PATCH 12/17] qcow2: Forward ZERO_WRITE flag for full preallocation

RH-Author: Kevin Wolf <kwolf@redhat.com>
Message-id: <20200608150140.38218-12-kwolf@redhat.com>
Patchwork-id: 97456
O-Subject: [RHEL-AV-8.2.1 qemu-kvm PATCH 11/11] qcow2: Forward ZERO_WRITE flag for full preallocation
Bugzilla: 1780574
RH-Acked-by: Sergio Lopez Pascual <slp@redhat.com>
RH-Acked-by: Eric Blake <eblake@redhat.com>
RH-Acked-by: Max Reitz <mreitz@redhat.com>

The BDRV_REQ_ZERO_WRITE is currently implemented in a way that first the
image is possibly preallocated and then the zero flag is added to all
clusters. This means that a copy-on-write operation may be needed when
writing to these clusters, despite having used preallocation, negating
one of the major benefits of preallocation.

Instead, try to forward the BDRV_REQ_ZERO_WRITE to the protocol driver,
and if the protocol driver can ensure that the new area reads as zeros,
we can skip setting the zero flag in the qcow2 layer.

Unfortunately, the same approach doesn't work for metadata
preallocation, so we'll still set the zero flag there.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200424142701.67053-1-kwolf@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit eb8a0cf3ba26611f3981f8f45ac6a868975a68cc)
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>
---
 block/qcow2.c              | 22 +++++++++++++++++++---
 tests/qemu-iotests/274.out |  4 ++--
 2 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index f3d6cb0..b783662 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -4153,9 +4153,25 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
         /* Allocate the data area */
         new_file_size = allocation_start +
                         nb_new_data_clusters * s->cluster_size;
-        /* Image file grows, so @exact does not matter */
-        ret = bdrv_co_truncate(bs->file, new_file_size, false, prealloc, 0,
-                               errp);
+        /*
+         * Image file grows, so @exact does not matter.
+         *
+         * If we need to zero out the new area, try first whether the protocol
+         * driver can already take care of this.
+         */
+        if (flags & BDRV_REQ_ZERO_WRITE) {
+            ret = bdrv_co_truncate(bs->file, new_file_size, false, prealloc,
+                                   BDRV_REQ_ZERO_WRITE, NULL);
+            if (ret >= 0) {
+                flags &= ~BDRV_REQ_ZERO_WRITE;
+            }
+        } else {
+            ret = -1;
+        }
+        if (ret < 0) {
+            ret = bdrv_co_truncate(bs->file, new_file_size, false, prealloc, 0,
+                                   errp);
+        }
         if (ret < 0) {
             error_prepend(errp, "Failed to resize underlying file: ");
             qcow2_free_clusters(bs, allocation_start,
diff --git a/tests/qemu-iotests/274.out b/tests/qemu-iotests/274.out
index 1a796fd..9d6fdeb 100644
--- a/tests/qemu-iotests/274.out
+++ b/tests/qemu-iotests/274.out
@@ -187,7 +187,7 @@ read 65536/65536 bytes at offset 9437184
 10 MiB (0xa00000) bytes     allocated at offset 5 MiB (0x500000)
 
 [{ "start": 0, "length": 5242880, "depth": 1, "zero": true, "data": false},
-{ "start": 5242880, "length": 10485760, "depth": 0, "zero": true, "data": false, "offset": 327680}]
+{ "start": 5242880, "length": 10485760, "depth": 0, "zero": false, "data": true, "offset": 327680}]
 
 === preallocation=full ===
 Formatting 'TEST_DIR/PID-base', fmt=qcow2 size=16777216 cluster_size=65536 lazy_refcounts=off refcount_bits=16
@@ -206,7 +206,7 @@ read 65536/65536 bytes at offset 11534336
 4 MiB (0x400000) bytes     allocated at offset 8 MiB (0x800000)
 
 [{ "start": 0, "length": 8388608, "depth": 1, "zero": true, "data": false},
-{ "start": 8388608, "length": 4194304, "depth": 0, "zero": true, "data": false, "offset": 327680}]
+{ "start": 8388608, "length": 4194304, "depth": 0, "zero": false, "data": true, "offset": 327680}]
 
 === preallocation=off ===
 Formatting 'TEST_DIR/PID-base', fmt=qcow2 size=393216 cluster_size=65536 lazy_refcounts=off refcount_bits=16
-- 
1.8.3.1