Re: [HMM 12/15] mm/migrate: new memory migration helper for use with device memory v4

[Jerome Glisse]

On Fri, Jun 30, 2017 at 04:19:25PM -0700, Evgeny Baskakov wrote:
> Hi Jerome,
> 
> It seems that the kernel can pass 0 in src_pfns for pages that it cannot
> migrate (i.e. the kernel knows that they cannot migrate prior to calling
> alloc_and_copy).
> 
> So, a zero in src_pfns can mean either "the page is not allocated yet" or
> "the page cannot migrate".
> 
> Can migrate_vma set the MIGRATE_PFN_MIGRATE flag for not allocated pages? On
> the driver side it is difficult to differentiate between the cases.

So this is what is happening in v24. For thing that can not be migrated you
get 0 and for things that are not allocated you get MIGRATE_PFN_MIGRATE like
the updated comments in migrate.h explain.

Cheers,
Jérôme