Re: [PATCH] migration: Move bitmap_mutex out of migration_bitmap_clear_dirty()

[Peter Xu <peterx@redhat.com>]

On Thu, Jul 08, 2021 at 02:55:23AM +0000, Wang, Wei W wrote:
> On Thursday, July 8, 2021 12:55 AM, Peter Xu wrote:
> > On Wed, Jul 07, 2021 at 08:34:50AM +0000, Wang, Wei W wrote:
> > > On Wednesday, July 7, 2021 1:47 AM, Peter Xu wrote:
> > > > On Sat, Jul 03, 2021 at 02:53:27AM +0000, Wang, Wei W wrote:
> > > > > +       do {
> > > > > +            page_to_clear = start + (i++ <<
> > > > > + block->clear_bmap_shift);
> > > >
> > > > Why "i" needs to be shifted?
> > >
> > > Just move to the next clear chunk, no?
> > > For example, (1 << 18) pages chunk (i.e. 1GB).
> > 
> > But migration_clear_memory_region_dirty_bitmap() has done the shifting?
> > 
> 
> Please see this example: start=0, npages = 2 * (1 <<18), i.e. we have 2 chunks of pages to clear, and start from 0.
> First chunk: from 0 to (1 <<18);
> Second chunk: from (1 << 18) to 2*(1<<18).
> 
> To clear the second chunk, we need to pass (start + "1 << 18") to migration_clear_memory_region_dirty_bitmap(),
> and clear_bmap_test_and_clear() there will do ">>18" to transform it into the id of clear_bitmap, which is 1.

I see what you meant, thanks; however I still think it's wrong.

+       do {
+            page_to_clear = start + (i++ << block->clear_bmap_shift);
+            migration_clear_memory_region_dirty_bitmap(ram_state,
+                                                       block,
+                                                       page_to_clear);
+       } while (i <= npages >> block->clear_bmap_shift);.

Consider start=1G-1M, npages=2M, this code will only clear log for the 1st 1G
chunk, while we need to clear both the 1st and 2nd 1G chunk.

If we want to go this route, I think a helper would be great to clear-log for a
range of memory too.

Thanks,

-- 
Peter Xu