RE: [PATCH v1] migration: clear the memory region dirty bitmap when skipping free pages

["Wang, Wei W" <wei.w.wang@intel.com>]

On Wednesday, July 14, 2021 6:30 PM, David Hildenbrand wrote:
> 
> On 14.07.21 12:27, Michael S. Tsirkin wrote:
> > On Wed, Jul 14, 2021 at 03:51:04AM -0400, Wei Wang wrote:
> >> When skipping free pages, their corresponding dirty bits in the
> >> memory region dirty bitmap need to be cleared. Otherwise the skipped
> >> pages will be sent in the next round after the migration thread syncs
> >> dirty bits from the memory region dirty bitmap.
> >>
> >> migration_clear_memory_region_dirty_bitmap_range is put outside the
> >> bitmap_mutex, becasue
> >
> > because?
> >
> >> memory_region_clear_dirty_bitmap is possible to block on the kvm slot
> >> mutex (don't want holding bitmap_mutex while blocked on another
> >> mutex), and clear_bmap_test_and_clear uses atomic operation.
> 
> How is that different from our existing caller?
> 
> Please either clean everything up, completely avoiding the lock (separate
> patch), or move it under the lock.
> 
> Or am I missing something important?

That seems ok to me and Peter to have it outside the lock. Not sure if Dave or Juan knows the reason why clear_bmap needs to be under the mutex given that it is atomic operation.

Best,
Wei