[pve-devel] [RFC qemu] migration/block-dirty-bitmap: migrate other bitmaps even if one fails

[Stefan Reiter <s.reiter@proxmox.com>]

If the checks in bdrv_dirty_bitmap_check fail, that only means that this
one specific bitmap cannot be migrated. That is not an error condition
for any other bitmaps on the same block device.

Fixes dirty-bitmap migration with sync=bitmap, as the bitmaps used for
that are obviously marked as "busy", which would cause none at all to be
transferred.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
---

NOTE: This is more or less a required workaround. The correct solution would
probably be to somehow exclude the bitmap used for sync=bitmap for migration
purposes, or handle it correctly otherwise.

ALSO: This does *not* fix the original bug, wherein QEMU segfaults on the source
side in sync=bitmap mode if any of the migration checks after the drive-mirror
fail. What this patch does is remove the error condition (which makes sense
either way), but if there is a different error, I'd still expect it to SEGV.

@Fabian: Potentially a bug in error handling from the bitmap sync mode patches
you carried? I'll keep looking too.

Crash is easily reproducible, just live-migrate a VM that has a replicated drive
on 5.1.0-4. 'add_bitmaps_to_list' will fail, and then QEMU segfaults with the
following stack trace (ioc=0 is the fault):

#0    0x0000555555de8b2f in qio_channel_detach_aio_context (ioc=0x0) at io/channel.c:452
#1    0x0000555555d91bd2 in nbd_client_detach_aio_context (bs=0x7fff879e91c0) at block/nbd.c:146
#2    0x0000555555d05050 in bdrv_detach_aio_context (bs=0x7fff879e91c0) at block.c:6267
#3    0x0000555555d05347 in bdrv_set_aio_context_ignore (bs=0x7fff879e91c0, new_context=0x7fffea332500, ignore=0x7fffffffcdc0) at block.c:6346
#4    0x0000555555d056f7 in bdrv_child_try_set_aio_context (bs=0x7fff879e91c0, ctx=0x7fffea332500, ignore_child=0x0, errp=0x0) at block.c:6450
#5    0x0000555555d0574e in bdrv_try_set_aio_context (bs=0x7fff879e91c0, ctx=0x7fffea332500, errp=0x0) at block.c:6459
#6    0x0000555555cfced7 in bdrv_replace_child (child=0x7fffea2cd580, new_bs=0x0) at block.c:2654
#7    0x0000555555cfd35e in bdrv_detach_child (child=0x7fffea2cd580) at block.c:2773
#8    0x0000555555cfd3a0 in bdrv_root_unref_child (child=0x7fffea2cd580) at block.c:2784
#9    0x0000555555d06f5b in block_job_remove_all_bdrv (job=0x7fffea896500) at blockjob.c:191
#10   0x0000555555d7c963 in mirror_exit_common (job=0x7fffea896500) at block/mirror.c:743
#11   0x0000555555d7cb02 in mirror_abort (job=0x7fffea896500) at block/mirror.c:783
#12   0x0000555555d0971a in job_abort (job=0x7fffea896500) at job.c:692
#13   0x0000555555d097be in job_finalize_single (job=0x7fffea896500) at job.c:713
#14   0x0000555555d09a47 in job_completed_txn_abort (job=0x7fffea896500) at job.c:791
#15   0x0000555555d09dc0 in job_completed (job=0x7fffea896500) at job.c:888
#16   0x0000555555d09e21 in job_exit (opaque=0x7fffea896500) at job.c:910
#17   0x0000555555e676fc in aio_bh_call (bh=0x7fffe8c141e0) at util/async.c:136
#18   0x0000555555e67806 in aio_bh_poll (ctx=0x7fffea332500) at util/async.c:164
#19   0x0000555555e51766 in aio_dispatch (ctx=0x7fffea332500) at util/aio-posix.c:380
#20   0x0000555555e67c39 in aio_ctx_dispatch (source=0x7fffea332500, callback=0x0, user_data=0x0) at util/async.c:306
#21   0x00007ffff7bf0f2e in g_main_context_dispatch + 0x2ae () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#22   0x0000555555e6ff77 in glib_pollfds_poll () at util/main-loop.c:217
#23   0x0000555555e6fff1 in os_host_main_loop_wait (timeout=0xd53f4) at util/main-loop.c:240
#24   0x0000555555e700f6 in main_loop_wait (nonblocking=0x0) at util/main-loop.c:516


 migration/block-dirty-bitmap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/migration/block-dirty-bitmap.c b/migration/block-dirty-bitmap.c
index 5bf0d9fbc6..1070c19181 100644
--- a/migration/block-dirty-bitmap.c
+++ b/migration/block-dirty-bitmap.c
@@ -323,7 +323,7 @@ static int add_bitmaps_to_list(DBMSaveState *s, BlockDriverState *bs,
 
         if (bdrv_dirty_bitmap_check(bitmap, BDRV_BITMAP_DEFAULT, &local_err)) {
             error_report_err(local_err);
-            return -1;
+            continue;
         }
 
         bdrv_ref(bs);
-- 
2.20.1