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migration: Fix possible infinite loop of ram save process 

When starting ram saving procedure (especially at the completion phase),
always set last_seen_block to non-NULL to make sure we can always correctly
detect the case where "we've migrated all the dirty pages".

Then we'll guarantee both last_seen_block and pss.block will be valid
always before the loop starts.

See the comment in the code for some details.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
This commit is contained in:
Peter Xu 2022-10-04 14:24:26 -04:00 committed by Juan Quintela
parent 4cc47b4395
commit 4934a5dd7c
1 changed files with 12 additions and 4 deletions
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16
migration/ram.c
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@ -2546,14 +2546,22 @@ static int ram_find_and_save_block(RAMState *rs)
return pages;
}
/*
* Always keep last_seen_block/last_page valid during this procedure,
* because find_dirty_block() relies on these values (e.g., we compare
* last_seen_block with pss.block to see whether we searched all the
* ramblocks) to detect the completion of migration. Having NULL value
* of last_seen_block can conditionally cause below loop to run forever.
*/
if (!rs->last_seen_block) {
rs->last_seen_block = QLIST_FIRST_RCU(&ram_list.blocks);
rs->last_page = 0;
}
pss.block = rs->last_seen_block;
pss.page = rs->last_page;
pss.complete_round = false;
if (!pss.block) {
pss.block = QLIST_FIRST_RCU(&ram_list.blocks);
}
do {
again = true;
found = get_queued_page(rs, &pss);