[PATCH v2 1/7] block-copy: streamline choice of copy_range vs. read/write

[Emanuele Giuseppe Esposito]

From: Paolo Bonzini <pbonzini@redhat.com>

Put the logic to determine the copy size in a separate function, so
that there is a simple state machine for the possible methods of
copying data from one BlockDriverState to the other.

While at it, store the common computation of block_copy_max_transfer
into a new field of BlockCopyState, and make sure that we always
obey max_transfer; that's more efficient even for the
COPY_RANGE_READ_WRITE case.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 block/block-copy.c | 59 ++++++++++++++++++++++++++++++----------------
 1 file changed, 39 insertions(+), 20 deletions(-)

diff --git a/block/block-copy.c b/block/block-copy.c
index 37c8e8504b..10ce51a244 100644
--- a/block/block-copy.c
+++ b/block/block-copy.c
@@ -28,6 +28,13 @@
 #define BLOCK_COPY_MAX_WORKERS 64
 #define BLOCK_COPY_SLICE_TIME 100000000ULL /* ns */
 
+typedef enum {
+    COPY_READ_WRITE_CLUSTER,
+    COPY_READ_WRITE,
+    COPY_RANGE_SMALL,
+    COPY_RANGE_FULL
+} BlockCopyMethod;
+
 static coroutine_fn int block_copy_task_entry(AioTask *task);
 
 typedef struct BlockCopyCallState {
@@ -85,8 +92,8 @@ typedef struct BlockCopyState {
     BdrvDirtyBitmap *copy_bitmap;
     int64_t in_flight_bytes;
     int64_t cluster_size;
-    bool use_copy_range;
-    int64_t copy_size;
+    BlockCopyMethod method;
+    int64_t max_transfer;
     uint64_t len;
     QLIST_HEAD(, BlockCopyTask) tasks; /* All tasks from all block-copy calls 
*/
     QLIST_HEAD(, BlockCopyCallState) calls;
@@ -148,6 +155,23 @@ static bool coroutine_fn 
block_copy_wait_one(BlockCopyState *s, int64_t offset,
     return true;
 }
 
+static inline int64_t block_copy_chunk_size(BlockCopyState *s)
+{
+    switch (s->method) {
+    case COPY_READ_WRITE_CLUSTER:
+        return s->cluster_size;
+    case COPY_READ_WRITE:
+    case COPY_RANGE_SMALL:
+        return MIN(MAX(s->cluster_size, BLOCK_COPY_MAX_BUFFER),
+                   s->max_transfer);
+    case COPY_RANGE_FULL:
+        return MIN(MAX(s->cluster_size, BLOCK_COPY_MAX_COPY_RANGE),
+                   s->max_transfer);
+    default:
+        abort();
+    }
+}
+
 /*
  * Search for the first dirty area in offset/bytes range and create task at
  * the beginning of it.
@@ -157,8 +181,9 @@ static BlockCopyTask *block_copy_task_create(BlockCopyState 
*s,
                                              int64_t offset, int64_t bytes)
 {
     BlockCopyTask *task;
-    int64_t max_chunk = MIN_NON_ZERO(s->copy_size, call_state->max_chunk);
+    int64_t max_chunk = block_copy_chunk_size(s);
 
+    max_chunk = MIN_NON_ZERO(max_chunk, call_state->max_chunk);
     if (!bdrv_dirty_bitmap_next_dirty_area(s->copy_bitmap,
                                            offset, offset + bytes,
                                            max_chunk, &offset, &bytes))
@@ -265,28 +290,27 @@ BlockCopyState *block_copy_state_new(BdrvChild *source, 
BdrvChild *target,
         .len = bdrv_dirty_bitmap_size(copy_bitmap),
         .write_flags = write_flags,
         .mem = shres_create(BLOCK_COPY_MAX_MEM),
+        .max_transfer = QEMU_ALIGN_DOWN(block_copy_max_transfer(source, target)
+                                        , cluster_size),
     };
 
-    if (block_copy_max_transfer(source, target) < cluster_size) {
+    if (s->max_transfer < cluster_size) {
         /*
          * copy_range does not respect max_transfer. We don't want to bother
          * with requests smaller than block-copy cluster size, so fallback to
          * buffered copying (read and write respect max_transfer on their
          * behalf).
          */
-        s->use_copy_range = false;
-        s->copy_size = cluster_size;
+        s->method = COPY_READ_WRITE_CLUSTER;
     } else if (write_flags & BDRV_REQ_WRITE_COMPRESSED) {
         /* Compression supports only cluster-size writes and no copy-range. */
-        s->use_copy_range = false;
-        s->copy_size = cluster_size;
+        s->method = COPY_READ_WRITE_CLUSTER;
     } else {
         /*
          * We enable copy-range, but keep small copy_size, until first
          * successful copy_range (look at block_copy_do_copy).
          */
-        s->use_copy_range = use_copy_range;
-        s->copy_size = MAX(s->cluster_size, BLOCK_COPY_MAX_BUFFER);
+        s->method = use_copy_range ? COPY_RANGE_SMALL : COPY_READ_WRITE;
     }
 
     ratelimit_init(&s->rate_limit);
@@ -369,30 +393,25 @@ static int coroutine_fn block_copy_do_copy(BlockCopyState 
*s,
         return ret;
     }
 
-    if (s->use_copy_range) {
+    if (s->method >= COPY_RANGE_SMALL) {
         ret = bdrv_co_copy_range(s->source, offset, s->target, offset, nbytes,
                                  0, s->write_flags);
         if (ret < 0) {
             trace_block_copy_copy_range_fail(s, offset, ret);
-            s->use_copy_range = false;
-            s->copy_size = MAX(s->cluster_size, BLOCK_COPY_MAX_BUFFER);
+            s->method = COPY_READ_WRITE;
             /* Fallback to read+write with allocated buffer */
         } else {
-            if (s->use_copy_range) {
+            if (s->method == COPY_RANGE_SMALL) {
                 /*
                  * Successful copy-range. Now increase copy_size.  copy_range
                  * does not respect max_transfer (it's a TODO), so we factor
                  * that in here.
                  *
-                 * Note: we double-check s->use_copy_range for the case when
+                 * Note: we double-check s->method for the case when
                  * parallel block-copy request unsets it during previous
                  * bdrv_co_copy_range call.
                  */
-                s->copy_size =
-                        MIN(MAX(s->cluster_size, BLOCK_COPY_MAX_COPY_RANGE),
-                            QEMU_ALIGN_DOWN(block_copy_max_transfer(s->source,
-                                                                    s->target),
-                                            s->cluster_size));
+                s->method = COPY_RANGE_FULL;
             }
             goto out;
         }
-- 
2.30.2