[Qemu-devel] [PATCH v3 6/7] block/backup: teach backup_cow_with_bounce_buffer to copy more at once

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

backup_cow_with_offload can transfer more than one cluster. Let
backup_cow_with_bounce_buffer behave similarly. It reduces the number
of IO requests, since there is no need to copy cluster by cluster.

Logic around bounce_buffer allocation changed: we can't just allocate
one-cluster-sized buffer to share for all iterations. We can't also
allocate buffer of full-request length it may be too large, so
BACKUP_MAX_BOUNCE_BUFFER is introduced. And finally, allocation logic
is to allocate a buffer sufficient to handle all remaining iterations
at the point where we need the buffer for the first time.

Bonus: get rid of pointer-to-pointer.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/backup.c | 65 +++++++++++++++++++++++++++++++-------------------
 1 file changed, 41 insertions(+), 24 deletions(-)

diff --git a/block/backup.c b/block/backup.c
index d482d93458..65f7212c85 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -27,6 +27,7 @@
 #include "qemu/error-report.h"
 
 #define BACKUP_CLUSTER_SIZE_DEFAULT (1 << 16)
+#define BACKUP_MAX_BOUNCE_BUFFER (64 * 1024 * 1024)
 
 typedef struct CowRequest {
     int64_t start_byte;
@@ -98,44 +99,55 @@ static void cow_request_end(CowRequest *req)
     qemu_co_queue_restart_all(&req->wait_queue);
 }
 
-/* Copy range to target with a bounce buffer and return the bytes copied. If
- * error occurred, return a negative error number */
+/*
+ * Copy range to target with a bounce buffer and return the bytes copied. If
+ * error occurred, return a negative error number
+ *
+ * @bounce_buffer is assumed to enough to store
+ * MIN(BACKUP_MAX_BOUNCE_BUFFER, @end - @start) bytes
+ */
 static int coroutine_fn backup_cow_with_bounce_buffer(BackupBlockJob *job,
                                                       int64_t start,
                                                       int64_t end,
                                                       bool is_write_notifier,
                                                       bool *error_is_read,
-                                                      void **bounce_buffer)
+                                                      void *bounce_buffer)
 {
     int ret;
     BlockBackend *blk = job->common.blk;
-    int nbytes;
+    int nbytes, remaining_bytes;
     int read_flags = is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0;
 
     assert(QEMU_IS_ALIGNED(start, job->cluster_size));
-    bdrv_reset_dirty_bitmap(job->copy_bitmap, start, job->cluster_size);
-    nbytes = MIN(job->cluster_size, job->len - start);
-    if (!*bounce_buffer) {
-        *bounce_buffer = blk_blockalign(blk, job->cluster_size);
-    }
+    bdrv_reset_dirty_bitmap(job->copy_bitmap, start, end - start);
+    nbytes = MIN(end - start, job->len - start);
 
-    ret = blk_co_pread(blk, start, nbytes, *bounce_buffer, read_flags);
-    if (ret < 0) {
-        trace_backup_do_cow_read_fail(job, start, ret);
-        if (error_is_read) {
-            *error_is_read = true;
+
+    remaining_bytes = nbytes;
+    while (remaining_bytes) {
+        int chunk = MIN(BACKUP_MAX_BOUNCE_BUFFER, remaining_bytes);
+
+        ret = blk_co_pread(blk, start, chunk, bounce_buffer, read_flags);
+        if (ret < 0) {
+            trace_backup_do_cow_read_fail(job, start, ret);
+            if (error_is_read) {
+                *error_is_read = true;
+            }
+            goto fail;
         }
-        goto fail;
-    }
 
-    ret = blk_co_pwrite(job->target, start, nbytes, *bounce_buffer,
-                        job->write_flags);
-    if (ret < 0) {
-        trace_backup_do_cow_write_fail(job, start, ret);
-        if (error_is_read) {
-            *error_is_read = false;
+        ret = blk_co_pwrite(job->target, start, chunk, bounce_buffer,
+                            job->write_flags);
+        if (ret < 0) {
+            trace_backup_do_cow_write_fail(job, start, ret);
+            if (error_is_read) {
+                *error_is_read = false;
+            }
+            goto fail;
         }
-        goto fail;
+
+        start += chunk;
+        remaining_bytes -= chunk;
     }
 
     return nbytes;
@@ -301,9 +313,14 @@ static int coroutine_fn backup_do_cow(BackupBlockJob *job,
             }
         }
         if (!job->use_copy_range) {
+            if (!bounce_buffer) {
+                size_t len = MIN(BACKUP_MAX_BOUNCE_BUFFER,
+                                 MAX(dirty_end - start, end - dirty_end));
+                bounce_buffer = blk_try_blockalign(job->common.blk, len);
+            }
             ret = backup_cow_with_bounce_buffer(job, start, dirty_end,
                                                 is_write_notifier,
-                                                error_is_read, &bounce_buffer);
+                                                error_is_read, bounce_buffer);
         }
         if (ret < 0) {
             break;
-- 
2.18.0