On 07/12/22 10:42, Dr. David Alan Gilbert wrote: > * Mike Kravetz (mike.kravetz@xxxxxxxxxx) wrote: > > On 06/24/22 17:36, James Houghton wrote: > > > After high-granularity mapping, page table entries for HugeTLB pages can > > > be of any size/type. (For example, we can have a 1G page mapped with a > > > mix of PMDs and PTEs.) This struct is to help keep track of a HugeTLB > > > PTE after we have done a page table walk. > > > > This has been rolling around in my head. > > > > Will this first use case (live migration) actually make use of this > > 'mixed mapping' model where hugetlb pages could be mapped at the PUD, > > PMD and PTE level all within the same vma? I only understand the use > > case from a high level. But, it seems that we would want to only want > > to migrate PTE (or PMD) sized pages and not necessarily a mix. > > I suspect we would pick one size and use that size for all transfers > when in postcopy; not sure if there are any side cases though. > > > The only reason I ask is because the code might be much simpler if all > > mappings within a vma were of the same size. Of course, the > > performance/latency of converting a large mapping may be prohibitively > > expensive. > > Imagine we're migrating a few TB VM, backed by 1GB hugepages, I'm guessing it > would be nice to clean up the PTE/PMDs for split 1GB pages as they're > completed rather than having thousands of them for the whole VM. > (I'm not sure if that is already doable) Seems that would be doable by calling MADV_COLLAPSE for 1GB pages as they are completed. Thanks for information on post copy. -- Mike Kravetz
