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Re: [RFC PATCH v3 03/27] qcow2: Process QCOW2_CLUSTER_ZERO_ALLOC cluster
From: |
Max Reitz |
Subject: |
Re: [RFC PATCH v3 03/27] qcow2: Process QCOW2_CLUSTER_ZERO_ALLOC clusters in handle_copied() |
Date: |
Thu, 20 Feb 2020 16:19:33 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 |
I have no idea how I managed to forgot some style comments I wrote
during the review, but, anyway:
On 22.12.19 12:36, Alberto Garcia wrote:
> 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 importante difference after this change is that clusters found in
s/importante/important/
> handle_copied() may now require copy-on-write, but this will be anyway
> necessary once we add support for subclusters.
>
> Signed-off-by: Alberto Garcia <address@hidden>
> ---
> block/qcow2-cluster.c | 226 +++++++++++++++++++++++-------------------
> 1 file changed, 126 insertions(+), 100 deletions(-)
>
> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> index e078bddcc2..9387f15866 100644
> --- a/block/qcow2-cluster.c
> +++ b/block/qcow2-cluster.c
[...]
> @@ -1035,15 +1040,53 @@ 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)
> {
[...]
> + /* 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 from the first cluster */
s/from/of/
(Otherwise it sounds a bit like this is the same entry for all clusters)
> + 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 from the last cluster */
s/from/of/
> + 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) {
> @@ -1069,18 +1112,20 @@ 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). */
Not sure why Patchew hasn’t complained, but the current coding style
requires /* and */ to be on separate lines for multi-line comments.
Max
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