Re: [PATCH v1 01/15] mm: add zone device coherent type memory support

Alistair Popple
2022-05-12T03:11:21Z

Alex Sierra <alex.sierra@amd.com> writes:

[...]

> diff --git a/mm/rmap.c b/mm/rmap.c
> index fedb82371efe..d57102cd4b43 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1995,7 +1995,8 @@ void try_to_migrate(struct folio *folio, enum ttu_flags flags)
> TTU_SYNC)))
> return;
>
> - if (folio_is_zone_device(folio) && !folio_is_device_private(folio))
> + if (folio_is_zone_device(folio) &&
> + (!folio_is_device_private(folio) && !folio_is_device_coherent(folio)))
> return;
>
> /*

I vaguely recall commenting on this previously, or at least intending
to. In try_to_migrate_one() we have this:

if (folio_is_zone_device(folio)) {
unsigned long pfn = folio_pfn(folio);
swp_entry_t entry;
pte_t swp_pte;

/*
* Store the pfn of the page in a special migration
* pte. do_swap_page() will wait until the migration
* pte is removed and then restart fault handling.
*/
entry = pte_to_swp_entry(pteval);
if (is_writable_device_private_entry(entry))
entry = make_writable_migration_entry(pfn);
else
entry = make_readable_migration_entry(pfn);
swp_pte = swp_entry_to_pte(entry);

The check in try_to_migrate() guarantees that if folio_is_zone_device()
is true this must be a DEVICE_PRIVATE page and it treats it as such by
assuming there is a special device private swap entry there.

Relying on that assumption seems bad, and I have no idea why I didn't
just use is_device_private_page() originally but I think the fix is just
to change this to:

if (folio_is_device_private(folio))

And let DEVICE_COHERENT pages fall through to normal page processing.

- Alistair