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qcow2: Add subcluster support to qcow2_co_pwrite_zeroes()
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This works now at the subcluster level and pwrite_zeroes_alignment is
updated accordingly.

qcow2_cluster_zeroize() is turned into qcow2_subcluster_zeroize() with
the following changes:

   - The request can now be subcluster-aligned.

   - The cluster-aligned body of the request is still zeroized using
     zero_in_l2_slice() as before.

   - The subcluster-aligned head and tail of the request are zeroized
     with the new zero_l2_subclusters() function.

There is just one thing to take into account for a possible future
improvement: compressed clusters cannot be partially zeroized so
zero_l2_subclusters() on the head or the tail can return -ENOTSUP.
This makes the caller repeat the *complete* request and write actual
zeroes to disk. This is sub-optimal because

   1) if the head area was compressed we would still be able to use
      the fast path for the body and possibly the tail.

   2) if the tail area was compressed we are writing zeroes to the
      head and the body areas, which are already zeroized.

Signed-off-by: Alberto Garcia <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Message-Id: <17e05e2ee7e12f10dcf012da81e83ebe27eb3bef.1594396418.git.berto@igalia.com>
Signed-off-by: Max Reitz <[email protected]>
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@bertogg @XanClic
bertogg authored and XanClic committed Aug 25, 2020
1 parent bf4a66e commit a6841a2
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Showing 3 changed files with 94 additions and 24 deletions.
81 changes: 75 additions & 6 deletions block/qcow2-cluster.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -2016,12 +2016,59 @@ static int zero_in_l2_slice(BlockDriverState *bs, uint64_t offset,
return nb_clusters;
}

int qcow2_cluster_zeroize(BlockDriverState *bs, uint64_t offset,
uint64_t bytes, int flags)
static int zero_l2_subclusters(BlockDriverState *bs, uint64_t offset,
unsigned nb_subclusters)
{
BDRVQcow2State *s = bs->opaque;
uint64_t *l2_slice;
uint64_t old_l2_bitmap, l2_bitmap;
int l2_index, ret, sc = offset_to_sc_index(s, offset);

/* For full clusters use zero_in_l2_slice() instead */
assert(nb_subclusters > 0 && nb_subclusters < s->subclusters_per_cluster);
assert(sc + nb_subclusters <= s->subclusters_per_cluster);
assert(offset_into_subcluster(s, offset) == 0);

ret = get_cluster_table(bs, offset, &l2_slice, &l2_index);
if (ret < 0) {
return ret;
}

switch (qcow2_get_cluster_type(bs, get_l2_entry(s, l2_slice, l2_index))) {
case QCOW2_CLUSTER_COMPRESSED:
ret = -ENOTSUP; /* We cannot partially zeroize compressed clusters */
goto out;
case QCOW2_CLUSTER_NORMAL:
case QCOW2_CLUSTER_UNALLOCATED:
break;
default:
g_assert_not_reached();
}

old_l2_bitmap = l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index);

l2_bitmap |= QCOW_OFLAG_SUB_ZERO_RANGE(sc, sc + nb_subclusters);
l2_bitmap &= ~QCOW_OFLAG_SUB_ALLOC_RANGE(sc, sc + nb_subclusters);

if (old_l2_bitmap != l2_bitmap) {
set_l2_bitmap(s, l2_slice, l2_index, l2_bitmap);
qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
}

ret = 0;
out:
qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);

return ret;
}

int qcow2_subcluster_zeroize(BlockDriverState *bs, uint64_t offset,
uint64_t bytes, int flags)
{
BDRVQcow2State *s = bs->opaque;
uint64_t end_offset = offset + bytes;
uint64_t nb_clusters;
unsigned head, tail;
int64_t cleared;
int ret;

Expand All @@ -2036,8 +2083,8 @@ int qcow2_cluster_zeroize(BlockDriverState *bs, uint64_t offset,
}

/* Caller must pass aligned values, except at image end */
assert(QEMU_IS_ALIGNED(offset, s->cluster_size));
assert(QEMU_IS_ALIGNED(end_offset, s->cluster_size) ||
assert(offset_into_subcluster(s, offset) == 0);
assert(offset_into_subcluster(s, end_offset) == 0 ||
end_offset >= bs->total_sectors << BDRV_SECTOR_BITS);

/*
Expand All @@ -2052,11 +2099,26 @@ int qcow2_cluster_zeroize(BlockDriverState *bs, uint64_t offset,
return -ENOTSUP;
}

/* Each L2 slice is handled by its own loop iteration */
nb_clusters = size_to_clusters(s, bytes);
head = MIN(end_offset, ROUND_UP(offset, s->cluster_size)) - offset;
offset += head;

tail = (end_offset >= bs->total_sectors << BDRV_SECTOR_BITS) ? 0 :
end_offset - MAX(offset, start_of_cluster(s, end_offset));
end_offset -= tail;

s->cache_discards = true;

if (head) {
ret = zero_l2_subclusters(bs, offset - head,
size_to_subclusters(s, head));
if (ret < 0) {
goto fail;
}
}

/* Each L2 slice is handled by its own loop iteration */
nb_clusters = size_to_clusters(s, end_offset - offset);

while (nb_clusters > 0) {
cleared = zero_in_l2_slice(bs, offset, nb_clusters, flags);
if (cleared < 0) {
Expand All @@ -2068,6 +2130,13 @@ int qcow2_cluster_zeroize(BlockDriverState *bs, uint64_t offset,
offset += (cleared * s->cluster_size);
}

if (tail) {
ret = zero_l2_subclusters(bs, end_offset, size_to_subclusters(s, tail));
if (ret < 0) {
goto fail;
}
}

ret = 0;
fail:
s->cache_discards = false;
Expand Down
33 changes: 17 additions & 16 deletions block/qcow2.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1913,7 +1913,7 @@ static void qcow2_refresh_limits(BlockDriverState *bs, Error **errp)
/* Encryption works on a sector granularity */
bs->bl.request_alignment = qcrypto_block_get_sector_size(s->crypto);
}
bs->bl.pwrite_zeroes_alignment = s->cluster_size;
bs->bl.pwrite_zeroes_alignment = s->subcluster_size;
bs->bl.pdiscard_alignment = s->cluster_size;
}

Expand Down Expand Up @@ -3833,8 +3833,9 @@ static coroutine_fn int qcow2_co_pwrite_zeroes(BlockDriverState *bs,
int ret;
BDRVQcow2State *s = bs->opaque;

uint32_t head = offset % s->cluster_size;
uint32_t tail = (offset + bytes) % s->cluster_size;
uint32_t head = offset_into_subcluster(s, offset);
uint32_t tail = ROUND_UP(offset + bytes, s->subcluster_size) -
(offset + bytes);

trace_qcow2_pwrite_zeroes_start_req(qemu_coroutine_self(), offset, bytes);
if (offset + bytes == bs->total_sectors * BDRV_SECTOR_SIZE) {
Expand All @@ -3846,20 +3847,19 @@ static coroutine_fn int qcow2_co_pwrite_zeroes(BlockDriverState *bs,
unsigned int nr;
QCow2SubclusterType type;

assert(head + bytes <= s->cluster_size);
assert(head + bytes + tail <= s->subcluster_size);

/* check whether remainder of cluster already reads as zero */
if (!(is_zero(bs, offset - head, head) &&
is_zero(bs, offset + bytes,
tail ? s->cluster_size - tail : 0))) {
is_zero(bs, offset + bytes, tail))) {
return -ENOTSUP;
}

qemu_co_mutex_lock(&s->lock);
/* We can have new write after previous check */
offset = QEMU_ALIGN_DOWN(offset, s->cluster_size);
bytes = s->cluster_size;
nr = s->cluster_size;
offset -= head;
bytes = s->subcluster_size;
nr = s->subcluster_size;
ret = qcow2_get_host_offset(bs, offset, &nr, &off, &type);
if (ret < 0 ||
(type != QCOW2_SUBCLUSTER_UNALLOCATED_PLAIN &&
Expand All @@ -3875,8 +3875,8 @@ static coroutine_fn int qcow2_co_pwrite_zeroes(BlockDriverState *bs,

trace_qcow2_pwrite_zeroes(qemu_coroutine_self(), offset, bytes);

/* Whatever is left can use real zero clusters */
ret = qcow2_cluster_zeroize(bs, offset, bytes, flags);
/* Whatever is left can use real zero subclusters */
ret = qcow2_subcluster_zeroize(bs, offset, bytes, flags);
qemu_co_mutex_unlock(&s->lock);

return ret;
Expand Down Expand Up @@ -4357,15 +4357,16 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
}

if ((flags & BDRV_REQ_ZERO_WRITE) && offset > old_length) {
uint64_t zero_start = QEMU_ALIGN_UP(old_length, s->cluster_size);
uint64_t zero_start = QEMU_ALIGN_UP(old_length, s->subcluster_size);

/*
* Use zero clusters as much as we can. qcow2_cluster_zeroize()
* requires a cluster-aligned start. The end may be unaligned if it is
* at the end of the image (which it is here).
* Use zero clusters as much as we can. qcow2_subcluster_zeroize()
* requires a subcluster-aligned start. The end may be unaligned if
* it is at the end of the image (which it is here).
*/
if (offset > zero_start) {
ret = qcow2_cluster_zeroize(bs, zero_start, offset - zero_start, 0);
ret = qcow2_subcluster_zeroize(bs, zero_start, offset - zero_start,
0);
if (ret < 0) {
error_setg_errno(errp, -ret, "Failed to zero out new clusters");
goto fail;
Expand Down
4 changes: 2 additions & 2 deletions block/qcow2.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -898,8 +898,8 @@ void qcow2_alloc_cluster_abort(BlockDriverState *bs, QCowL2Meta *m);
int qcow2_cluster_discard(BlockDriverState *bs, uint64_t offset,
uint64_t bytes, enum qcow2_discard_type type,
bool full_discard);
int qcow2_cluster_zeroize(BlockDriverState *bs, uint64_t offset,
uint64_t bytes, int flags);
int qcow2_subcluster_zeroize(BlockDriverState *bs, uint64_t offset,
uint64_t bytes, int flags);

int qcow2_expand_zero_clusters(BlockDriverState *bs,
BlockDriverAmendStatusCB *status_cb,
Expand Down

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