Re: [Qemu-devel] [PATCH v2] migratioin/ram: leave RAMBlock->bmap blank on allocating

[Wei Yang <richardw.yang@linux.intel.com>]

On Tue, Jun 04, 2019 at 11:51:24AM +0800, Peter Xu wrote:
>On Tue, Jun 04, 2019 at 08:28:10AM +0800, Wei Yang wrote:
>> During migration, we would sync bitmap from ram_list.dirty_memory to
>> RAMBlock.bmap in cpu_physical_memory_sync_dirty_bitmap().
>> 
>> Since we set RAMBlock.bmap and ram_list.dirty_memory both to all 1, this
>> means at the first round this sync is meaningless and is a duplicated
>> work.
>> 
>> Leaving RAMBlock->bmap blank on allocating would have a side effect on
>> migration_dirty_pages, since it is calculated from the result of
>> cpu_physical_memory_sync_dirty_bitmap(). To keep it right, we need to
>> set migration_dirty_pages to 0 in ram_state_init().
>> 
>> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
>> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
>> 
>> ---
>> v2: add a comment explaining why leaving RAMBlock.bmap clear
>> ---
>>  migration/ram.c | 16 +++++++++-------
>>  1 file changed, 9 insertions(+), 7 deletions(-)
>> 
>> diff --git a/migration/ram.c b/migration/ram.c
>> index 4c60869226..e9a27ea5e6 100644
>> --- a/migration/ram.c
>> +++ b/migration/ram.c
>> @@ -3181,12 +3181,7 @@ static int ram_state_init(RAMState **rsp)
>>      qemu_mutex_init(&(*rsp)->src_page_req_mutex);
>>      QSIMPLEQ_INIT(&(*rsp)->src_page_requests);
>>  
>> -    /*
>> -     * Count the total number of pages used by ram blocks not including any
>> -     * gaps due to alignment or unplugs.
>> -     */
>> -    (*rsp)->migration_dirty_pages = ram_bytes_total() >> TARGET_PAGE_BITS;
>> -
>
>Since you've spent time discovering this relationship, we can also
>comment on this too to hint future readers:
>
>       /*
>        * This must match with the initial values of dirty bitmap.
>        * Currently we initialize the dirty bitmap to all zeros so
>        * here the total dirty page count is zero.
>        */
>

You are right.

>> +    (*rsp)->migration_dirty_pages = 0;
>>      ram_state_reset(*rsp);
>>  
>>      return 0;
>> @@ -3201,8 +3196,15 @@ static void ram_list_init_bitmaps(void)
>>      if (ram_bytes_total()) {
>>          RAMBLOCK_FOREACH_NOT_IGNORED(block) {
>>              pages = block->max_length >> TARGET_PAGE_BITS;
>> +            /*
>> +             * We leave RAMBlock.bmap clear here and they will be sync from
>> +             * ram_list.dirty_memory[DIRTY_MEMORY_MIGRATION] in
>> +             * migration_bitmap_sync_precopy().
>
>(This sentence is a bit confusing to me)
>
>> +             *
>> +             * ram_list.dirty_memory[DIRTY_MEMORY_MIGRATION] is set to all 1
>> +             * in ram_block_add().
>
>How about:
>
>    The initial dirty bitmap for migration must be set with all ones
>    to make sure we'll migrate every guest RAM page to destination.
>    Here we didn't set RAMBlock.bmap simply because it is already set
>    in ram_list.dirty_memory[DIRTY_MEMORY_MIGRATION] in ram_block_add,
>    and that's where we'll sync the dirty bitmaps.  Here setting
>    RAMBlock.bmap would be fine too but not necessary.
>
>?
>

Sounds better :-)

>> +             */
>>              block->bmap = bitmap_new(pages);
>> -            bitmap_set(block->bmap, 0, pages);
>>              if (migrate_postcopy_ram()) {
>>                  block->unsentmap = bitmap_new(pages);
>>                  bitmap_set(block->unsentmap, 0, pages);
>> -- 
>> 2.19.1
>> 
>
>Regards,
>
>-- 
>Peter Xu

-- 
Wei Yang
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