[Qemu-devel] [PATCH v2] mirror: follow AioContext change gracefully

[Fam Zheng <famz@redhat.com>]

From: Stefan Hajnoczi <stefanha@redhat.com>

When dataplane is enabled or disabled the drive switches to a new
AioContext.  The mirror block job must also move to the new AioContext
so that drive accesses are always made within its AioContext.

This patch partially achieves that by draining target and source
requests to reach a quiescent point.  The job is resumed in the new
AioContext after moving s->target into the new AioContext.

The quiesce_requested flag is added to deal with yield points in
block_job_sleep_ns(), bdrv_is_allocated_above(), and
bdrv_get_block_status_above().  Previously they continue executing in
the old AioContext. The nested aio_poll in mirror_detach_aio_context
will drive the mirror coroutine upto fixed yield points, where
mirror_check_for_quiesce is called.

Cc: Fam Zheng <famz@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Jeff Cody <jcody@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
[Drain source as well, and add s->quiesce_requested flag. -- Fam]
Signed-off-by: Fam Zheng <famz@redhat.com>

---

v2: Picked up Stefan's RFC patch and move on towards a more complete
fix.  Please review!

Jason: it would be nice if you could test this version again. It differs
from the previous version.
---
 block/mirror.c | 45 ++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 44 insertions(+), 1 deletion(-)

diff --git a/block/mirror.c b/block/mirror.c
index 80fd3c7..142199a 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -63,6 +63,8 @@ typedef struct MirrorBlockJob {
     int ret;
     bool unmap;
     bool waiting_for_io;
+    bool quiesce_requested; /* temporarily detached to move AioContext,
+                               don't do more I/O */
     int target_cluster_sectors;
     int max_iov;
 } MirrorBlockJob;
@@ -119,7 +121,7 @@ static void mirror_iteration_done(MirrorOp *op, int ret)
     qemu_iovec_destroy(&op->qiov);
     g_free(op);
 
-    if (s->waiting_for_io) {
+    if (s->waiting_for_io && !s->quiesce_requested) {
         qemu_coroutine_enter(s->common.co, NULL);
     }
 }
@@ -307,6 +309,14 @@ static void mirror_do_zero_or_discard(MirrorBlockJob *s,
     }
 }
 
+static void coroutine_fn mirror_check_for_quiesce(MirrorBlockJob *s)
+{
+    if (s->quiesce_requested) {
+        s->quiesce_requested = false;
+        qemu_coroutine_yield();
+    }
+}
+
 static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s)
 {
     BlockDriverState *source = blk_bs(s->common.blk);
@@ -331,6 +341,7 @@ static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s)
         mirror_wait_for_io(s);
     }
 
+    mirror_check_for_quiesce(s);
     /* Find the number of consective dirty chunks following the first dirty
      * one, and wait for in flight requests in them. */
     while (nb_chunks * sectors_per_chunk < (s->buf_size >> BDRV_SECTOR_BITS)) {
@@ -442,6 +453,31 @@ static void mirror_drain(MirrorBlockJob *s)
     }
 }
 
+static void mirror_attached_aio_context(AioContext *new_context, void *opaque)
+{
+    MirrorBlockJob *s = opaque;
+
+    blk_set_aio_context(s->target, new_context);
+
+    /* Resume execution */
+    assert(!s->quiesce_requested);
+    if (s->waiting_for_io) {
+        qemu_coroutine_enter(s->common.co, NULL);
+    }
+}
+
+static void mirror_detach_aio_context(void *opaque)
+{
+    MirrorBlockJob *s = opaque;
+
+    /* Complete pending write requests */
+    assert(!s->quiesce_requested);
+    s->quiesce_requested = true;
+    while (s->quiesce_requested || s->in_flight) {
+        aio_poll(blk_get_aio_context(s->common.blk), true);
+    }
+}
+
 typedef struct {
     int ret;
 } MirrorExitData;
@@ -491,6 +527,8 @@ static void mirror_exit(BlockJob *job, void *opaque)
     if (replace_aio_context) {
         aio_context_release(replace_aio_context);
     }
+    blk_remove_aio_context_notifier(s->common.blk, mirror_attached_aio_context,
+                                    mirror_detach_aio_context, s);
     g_free(s->replaces);
     bdrv_op_unblock_all(target_bs, s->common.blocker);
     blk_unref(s->target);
@@ -583,6 +621,7 @@ static void coroutine_fn mirror_run(void *opaque)
                 block_job_sleep_ns(&s->common, QEMU_CLOCK_REALTIME, 0);
             }
 
+            mirror_check_for_quiesce(s);
             if (block_job_is_cancelled(&s->common)) {
                 goto immediate_exit;
             }
@@ -612,6 +651,7 @@ static void coroutine_fn mirror_run(void *opaque)
             goto immediate_exit;
         }
 
+        mirror_check_for_quiesce(s);
         cnt = bdrv_get_dirty_count(s->dirty_bitmap);
         /* s->common.offset contains the number of bytes already processed so
          * far, cnt is the number of dirty sectors remaining and
@@ -851,6 +891,9 @@ static void mirror_start_job(BlockDriverState *bs, BlockDriverState *target,
 
     bdrv_op_block_all(target, s->common.blocker);
 
+    blk_add_aio_context_notifier(s->common.blk, mirror_attached_aio_context,
+                                 mirror_detach_aio_context, s);
+
     s->common.co = qemu_coroutine_create(mirror_run);
     trace_mirror_start(bs, s, s->common.co, opaque);
     qemu_coroutine_enter(s->common.co, s);
-- 
2.8.3