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qcow2: Process QCOW2_CLUSTER_ZERO_ALLOC clusters in handle_copied() 

When writing to a qcow2 file there are two functions that take a
virtual offset and return a host offset, possibly allocating new
clusters if necessary:

   - handle_copied() looks for normal data clusters that are already
     allocated and have a reference count of 1. In those clusters we
     can simply write the data and there is no need to perform any
     copy-on-write.

   - handle_alloc() looks for clusters that do need copy-on-write,
     either because they haven't been allocated yet, because their
     reference count is != 1 or because they are ZERO_ALLOC clusters.

The ZERO_ALLOC case is a bit special because those are clusters that
are already allocated and they could perfectly be dealt with in
handle_copied() (as long as copy-on-write is performed when required).

In fact, there is extra code specifically for them in handle_alloc()
that tries to reuse the existing allocation if possible and frees them
otherwise.

This patch changes the handling of ZERO_ALLOC clusters so the
semantics of these two functions are now like this:

   - handle_copied() looks for clusters that are already allocated and
     which we can overwrite (NORMAL and ZERO_ALLOC clusters with a
     reference count of 1).

   - handle_alloc() looks for clusters for which we need a new
     allocation (all other cases).

One important difference after this change is that clusters found
in handle_copied() may now require copy-on-write, but this will be
necessary anyway once we add support for subclusters.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <eb17fc938f6be7be2e8d8ff42763d2c19241f866.1594396418.git.berto@igalia.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
This commit is contained in:
Alberto Garcia 2020-07-10 18:12:47 +02:00 committed by Max Reitz
parent c1587d877e
commit 57538c864f
1 changed files with 138 additions and 112 deletions
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250
block/qcow2-cluster.c
View File

@ -1040,13 +1040,18 @@ void qcow2_alloc_cluster_abort(BlockDriverState *bs, QCowL2Meta *m)
/*
* For a given write request, create a new QCowL2Meta structure, add
* it to @m and the BDRVQcow2State.cluster_allocs list.
* it to @m and the BDRVQcow2State.cluster_allocs list. If the write
* request does not need copy-on-write or changes to the L2 metadata
* then this function does nothing.
*
* @host_cluster_offset points to the beginning of the first cluster.
*
* @guest_offset and @bytes indicate the offset and length of the
* request.
*
* @l2_slice contains the L2 entries of all clusters involved in this
* write request.
*
* If @keep_old is true it means that the clusters were already
* allocated and will be overwritten. If false then the clusters are
* new and we have to decrease the reference count of the old ones.
@ -1054,15 +1059,53 @@ void qcow2_alloc_cluster_abort(BlockDriverState *bs, QCowL2Meta *m)
static void calculate_l2_meta(BlockDriverState *bs,
uint64_t host_cluster_offset,
uint64_t guest_offset, unsigned bytes,
QCowL2Meta **m, bool keep_old)
uint64_t *l2_slice, QCowL2Meta **m, bool keep_old)
{
BDRVQcow2State *s = bs->opaque;
unsigned cow_start_from = 0;
int l2_index = offset_to_l2_slice_index(s, guest_offset);
uint64_t l2_entry;
unsigned cow_start_from, cow_end_to;
unsigned cow_start_to = offset_into_cluster(s, guest_offset);
unsigned cow_end_from = cow_start_to + bytes;
unsigned cow_end_to = ROUND_UP(cow_end_from, s->cluster_size);
unsigned nb_clusters = size_to_clusters(s, cow_end_from);
QCowL2Meta *old_m = *m;
QCow2ClusterType type;
assert(nb_clusters <= s->l2_slice_size - l2_index);
/* Return if there's no COW (all clusters are normal and we keep them) */
if (keep_old) {
int i;
for (i = 0; i < nb_clusters; i++) {
l2_entry = be64_to_cpu(l2_slice[l2_index + i]);
if (qcow2_get_cluster_type(bs, l2_entry) != QCOW2_CLUSTER_NORMAL) {
break;
}
}
if (i == nb_clusters) {
return;
}
}
/* Get the L2 entry of the first cluster */
l2_entry = be64_to_cpu(l2_slice[l2_index]);
type = qcow2_get_cluster_type(bs, l2_entry);
if (type == QCOW2_CLUSTER_NORMAL && keep_old) {
cow_start_from = cow_start_to;
} else {
cow_start_from = 0;
}
/* Get the L2 entry of the last cluster */
l2_entry = be64_to_cpu(l2_slice[l2_index + nb_clusters - 1]);
type = qcow2_get_cluster_type(bs, l2_entry);
if (type == QCOW2_CLUSTER_NORMAL && keep_old) {
cow_end_to = cow_end_from;
} else {
cow_end_to = ROUND_UP(cow_end_from, s->cluster_size);
}
*m = g_malloc0(sizeof(**m));
**m = (QCowL2Meta) {
@ -1088,18 +1131,22 @@ static void calculate_l2_meta(BlockDriverState *bs,
QLIST_INSERT_HEAD(&s->cluster_allocs, *m, next_in_flight);
}
/* Returns true if writing to a cluster requires COW */
static bool cluster_needs_cow(BlockDriverState *bs, uint64_t l2_entry)
/*
* Returns true if writing to the cluster pointed to by @l2_entry
* requires a new allocation (that is, if the cluster is unallocated
* or has refcount > 1 and therefore cannot be written in-place).
*/
static bool cluster_needs_new_alloc(BlockDriverState *bs, uint64_t l2_entry)
{
switch (qcow2_get_cluster_type(bs, l2_entry)) {
case QCOW2_CLUSTER_NORMAL:
case QCOW2_CLUSTER_ZERO_ALLOC:
if (l2_entry & QCOW_OFLAG_COPIED) {
return false;
}
case QCOW2_CLUSTER_UNALLOCATED:
case QCOW2_CLUSTER_COMPRESSED:
case QCOW2_CLUSTER_ZERO_PLAIN:
case QCOW2_CLUSTER_ZERO_ALLOC:
return true;
default:
abort();
@ -1107,20 +1154,38 @@ static bool cluster_needs_cow(BlockDriverState *bs, uint64_t l2_entry)
}
/*
* Returns the number of contiguous clusters that can be used for an allocating
* write, but require COW to be performed (this includes yet unallocated space,
* which must copy from the backing file)
* Returns the number of contiguous clusters that can be written to
* using one single write request, starting from @l2_index.
* At most @nb_clusters are checked.
*
* If @new_alloc is true this counts clusters that are either
* unallocated, or allocated but with refcount > 1 (so they need to be
* newly allocated and COWed).
*
* If @new_alloc is false this counts clusters that are already
* allocated and can be overwritten in-place (this includes clusters
* of type QCOW2_CLUSTER_ZERO_ALLOC).
*/
static int count_cow_clusters(BlockDriverState *bs, int nb_clusters,
uint64_t *l2_slice, int l2_index)
static int count_single_write_clusters(BlockDriverState *bs, int nb_clusters,
uint64_t *l2_slice, int l2_index,
bool new_alloc)
{
BDRVQcow2State *s = bs->opaque;
uint64_t l2_entry = be64_to_cpu(l2_slice[l2_index]);
uint64_t expected_offset = l2_entry & L2E_OFFSET_MASK;
int i;
for (i = 0; i < nb_clusters; i++) {
uint64_t l2_entry = be64_to_cpu(l2_slice[l2_index + i]);
if (!cluster_needs_cow(bs, l2_entry)) {
l2_entry = be64_to_cpu(l2_slice[l2_index + i]);
if (cluster_needs_new_alloc(bs, l2_entry) != new_alloc) {
break;
}
if (!new_alloc) {
if (expected_offset != (l2_entry & L2E_OFFSET_MASK)) {
break;
}
expected_offset += s->cluster_size;
}
}
assert(i <= nb_clusters);
@ -1191,10 +1256,10 @@ static int handle_dependencies(BlockDriverState *bs, uint64_t guest_offset,
}
/*
* Checks how many already allocated clusters that don't require a copy on
* write there are at the given guest_offset (up to *bytes). If *host_offset is
* not INV_OFFSET, only physically contiguous clusters beginning at this host
* offset are counted.
* Checks how many already allocated clusters that don't require a new
* allocation there are at the given guest_offset (up to *bytes).
* If *host_offset is not INV_OFFSET, only physically contiguous clusters
* beginning at this host offset are counted.
*
* Note that guest_offset may not be cluster aligned. In this case, the
* returned *host_offset points to exact byte referenced by guest_offset and
@ -1203,12 +1268,12 @@ static int handle_dependencies(BlockDriverState *bs, uint64_t guest_offset,
* Returns:
* 0: if no allocated clusters are available at the given offset.
* *bytes is normally unchanged. It is set to 0 if the cluster
* is allocated and doesn't need COW, but doesn't have the right
* physical offset.
* is allocated and can be overwritten in-place but doesn't have
* the right physical offset.
*
* 1: if allocated clusters that don't require a COW are available at
* the requested offset. *bytes may have decreased and describes
* the length of the area that can be written to.
* 1: if allocated clusters that can be overwritten in place are
* available at the requested offset. *bytes may have decreased
* and describes the length of the area that can be written to.
*
* -errno: in error cases
*/
@ -1217,7 +1282,7 @@ static int handle_copied(BlockDriverState *bs, uint64_t guest_offset,
{
BDRVQcow2State *s = bs->opaque;
int l2_index;
uint64_t cluster_offset;
uint64_t l2_entry, cluster_offset;
uint64_t *l2_slice;
uint64_t nb_clusters;
unsigned int keep_clusters;
@ -1238,7 +1303,8 @@ static int handle_copied(BlockDriverState *bs, uint64_t guest_offset,
l2_index = offset_to_l2_slice_index(s, guest_offset);
nb_clusters = MIN(nb_clusters, s->l2_slice_size - l2_index);
assert(nb_clusters <= INT_MAX);
/* Limit total byte count to BDRV_REQUEST_MAX_BYTES */
nb_clusters = MIN(nb_clusters, BDRV_REQUEST_MAX_BYTES >> s->cluster_bits);
/* Find L2 entry for the first involved cluster */
ret = get_cluster_table(bs, guest_offset, &l2_slice, &l2_index);
@ -1246,41 +1312,39 @@ static int handle_copied(BlockDriverState *bs, uint64_t guest_offset,
return ret;
}
cluster_offset = be64_to_cpu(l2_slice[l2_index]);
l2_entry = be64_to_cpu(l2_slice[l2_index]);
cluster_offset = l2_entry & L2E_OFFSET_MASK;
/* Check how many clusters are already allocated and don't need COW */
if (qcow2_get_cluster_type(bs, cluster_offset) == QCOW2_CLUSTER_NORMAL
&& (cluster_offset & QCOW_OFLAG_COPIED))
{
/* If a specific host_offset is required, check it */
bool offset_matches =
(cluster_offset & L2E_OFFSET_MASK) == *host_offset;
if (offset_into_cluster(s, cluster_offset & L2E_OFFSET_MASK)) {
qcow2_signal_corruption(bs, true, -1, -1, "Data cluster offset "
"%#llx unaligned (guest offset: %#" PRIx64
")", cluster_offset & L2E_OFFSET_MASK,
guest_offset);
if (!cluster_needs_new_alloc(bs, l2_entry)) {
if (offset_into_cluster(s, cluster_offset)) {
qcow2_signal_corruption(bs, true, -1, -1, "%s cluster offset "
"%#" PRIx64 " unaligned (guest offset: %#"
PRIx64 ")", l2_entry & QCOW_OFLAG_ZERO ?
"Preallocated zero" : "Data",
cluster_offset, guest_offset);
ret = -EIO;
goto out;
}
if (*host_offset != INV_OFFSET && !offset_matches) {
/* If a specific host_offset is required, check it */
if (*host_offset != INV_OFFSET && cluster_offset != *host_offset) {
*bytes = 0;
ret = 0;
goto out;
}
/* We keep all QCOW_OFLAG_COPIED clusters */
keep_clusters =
count_contiguous_clusters(bs, nb_clusters, s->cluster_size,
&l2_slice[l2_index],
QCOW_OFLAG_COPIED | QCOW_OFLAG_ZERO);
keep_clusters = count_single_write_clusters(bs, nb_clusters, l2_slice,
l2_index, false);
assert(keep_clusters <= nb_clusters);
*bytes = MIN(*bytes,
keep_clusters * s->cluster_size
- offset_into_cluster(s, guest_offset));
assert(*bytes != 0);
calculate_l2_meta(bs, cluster_offset, guest_offset,
*bytes, l2_slice, m, true);
ret = 1;
} else {
@ -1294,8 +1358,7 @@ out:
/* Only return a host offset if we actually made progress. Otherwise we
* would make requirements for handle_alloc() that it can't fulfill */
if (ret > 0) {
*host_offset = (cluster_offset & L2E_OFFSET_MASK)
+ offset_into_cluster(s, guest_offset);
*host_offset = cluster_offset + offset_into_cluster(s, guest_offset);
}
return ret;
@ -1356,9 +1419,10 @@ static int do_alloc_cluster_offset(BlockDriverState *bs, uint64_t guest_offset,
}
/*
* Allocates new clusters for an area that either is yet unallocated or needs a
* copy on write. If *host_offset is not INV_OFFSET, clusters are only
* allocated if the new allocation can match the specified host offset.
* Allocates new clusters for an area that is either still unallocated or
* cannot be overwritten in-place. If *host_offset is not INV_OFFSET,
* clusters are only allocated if the new allocation can match the specified
* host offset.
*
* Note that guest_offset may not be cluster aligned. In this case, the
* returned *host_offset points to exact byte referenced by guest_offset and
@ -1381,12 +1445,10 @@ static int handle_alloc(BlockDriverState *bs, uint64_t guest_offset,
BDRVQcow2State *s = bs->opaque;
int l2_index;
uint64_t *l2_slice;
uint64_t entry;
uint64_t nb_clusters;
int ret;
bool keep_old_clusters = false;
uint64_t alloc_cluster_offset = INV_OFFSET;
uint64_t alloc_cluster_offset;
trace_qcow2_handle_alloc(qemu_coroutine_self(), guest_offset, *host_offset,
*bytes);
@ -1401,10 +1463,8 @@ static int handle_alloc(BlockDriverState *bs, uint64_t guest_offset,
l2_index = offset_to_l2_slice_index(s, guest_offset);
nb_clusters = MIN(nb_clusters, s->l2_slice_size - l2_index);
assert(nb_clusters <= INT_MAX);
/* Limit total allocation byte count to INT_MAX */
nb_clusters = MIN(nb_clusters, INT_MAX >> s->cluster_bits);
/* Limit total allocation byte count to BDRV_REQUEST_MAX_BYTES */
nb_clusters = MIN(nb_clusters, BDRV_REQUEST_MAX_BYTES >> s->cluster_bits);
/* Find L2 entry for the first involved cluster */
ret = get_cluster_table(bs, guest_offset, &l2_slice, &l2_index);
@ -1412,67 +1472,32 @@ static int handle_alloc(BlockDriverState *bs, uint64_t guest_offset,
return ret;
}
entry = be64_to_cpu(l2_slice[l2_index]);
nb_clusters = count_cow_clusters(bs, nb_clusters, l2_slice, l2_index);
nb_clusters = count_single_write_clusters(bs, nb_clusters,
l2_slice, l2_index, true);
/* This function is only called when there were no non-COW clusters, so if
* we can't find any unallocated or COW clusters either, something is
* wrong with our code. */
assert(nb_clusters > 0);
if (qcow2_get_cluster_type(bs, entry) == QCOW2_CLUSTER_ZERO_ALLOC &&
(entry & QCOW_OFLAG_COPIED) &&
(*host_offset == INV_OFFSET ||
start_of_cluster(s, *host_offset) == (entry & L2E_OFFSET_MASK)))
{
int preallocated_nb_clusters;
if (offset_into_cluster(s, entry & L2E_OFFSET_MASK)) {
qcow2_signal_corruption(bs, true, -1, -1, "Preallocated zero "
"cluster offset %#llx unaligned (guest "
"offset: %#" PRIx64 ")",
entry & L2E_OFFSET_MASK, guest_offset);
ret = -EIO;
goto fail;
}
/* Try to reuse preallocated zero clusters; contiguous normal clusters
* would be fine, too, but count_cow_clusters() above has limited
* nb_clusters already to a range of COW clusters */
preallocated_nb_clusters =
count_contiguous_clusters(bs, nb_clusters, s->cluster_size,
&l2_slice[l2_index], QCOW_OFLAG_COPIED);
assert(preallocated_nb_clusters > 0);
nb_clusters = preallocated_nb_clusters;
alloc_cluster_offset = entry & L2E_OFFSET_MASK;
/* We want to reuse these clusters, so qcow2_alloc_cluster_link_l2()
* should not free them. */
keep_old_clusters = true;
/* Allocate at a given offset in the image file */
alloc_cluster_offset = *host_offset == INV_OFFSET ? INV_OFFSET :
start_of_cluster(s, *host_offset);
ret = do_alloc_cluster_offset(bs, guest_offset, &alloc_cluster_offset,
&nb_clusters);
if (ret < 0) {
goto out;
}
qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
if (alloc_cluster_offset == INV_OFFSET) {
/* Allocate, if necessary at a given offset in the image file */
alloc_cluster_offset = *host_offset == INV_OFFSET ? INV_OFFSET :
start_of_cluster(s, *host_offset);
ret = do_alloc_cluster_offset(bs, guest_offset, &alloc_cluster_offset,
&nb_clusters);
if (ret < 0) {
goto fail;
}
/* Can't extend contiguous allocation */
if (nb_clusters == 0) {
*bytes = 0;
return 0;
}
assert(alloc_cluster_offset != INV_OFFSET);
/* Can't extend contiguous allocation */
if (nb_clusters == 0) {
*bytes = 0;
ret = 0;
goto out;
}
assert(alloc_cluster_offset != INV_OFFSET);
/*
* Save info needed for meta data update.
*
@ -1495,13 +1520,14 @@ static int handle_alloc(BlockDriverState *bs, uint64_t guest_offset,
*bytes = MIN(*bytes, nb_bytes - offset_into_cluster(s, guest_offset));
assert(*bytes != 0);
calculate_l2_meta(bs, alloc_cluster_offset, guest_offset, *bytes,
m, keep_old_clusters);
calculate_l2_meta(bs, alloc_cluster_offset, guest_offset, *bytes, l2_slice,
m, false);
return 1;
ret = 1;
fail:
if (*m && (*m)->nb_clusters > 0) {
out:
qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
if (ret < 0 && *m && (*m)->nb_clusters > 0) {
QLIST_REMOVE(*m, next_in_flight);
}
return ret;