On Sat, Aug 17, 2019 at 01:31:28PM +0200, Christoph Hellwig wrote:

On Fri, Aug 16, 2019 at 05:11:07PM +0000, Jason Gunthorpe wrote:

...

• if (args->cpages)

• migrate_vma_prepare(args);
  

• if (args->cpages)

• migrate_vma_unmap(args);
  

• if (!args->cpages)

• return 0;
  

• migrate_vma_prepare(args);
• migrate_vma_unmap(args);

I don't think this is ok. Both migrate_vma_prepare and migrate_vma_unmap can reduce args->cpages, including possibly to 0.

Oh, yes, that was far too hasty on my part, I had assumed collect set the cpages. Thank you for checking

Jason