Re: [PATCH v1 0/3] udmabuf: Add support for page migration out of movable zone or CMA

[Jason Gunthorpe <jgg@nvidia.com>]

On Sun, Aug 27, 2023 at 07:05:59PM +0000, Kasireddy, Vivek wrote:
> Hi Jason, David,
> 
> > > > Sure, we can simply always fail when we detect ZONE_MOVABLE or
> > > MIGRATE_CMA.
> > > > Maybe that keeps at least some use cases working.
> > >
> > > That seems fairly reasonable
> > AFAICS, failing udmabuf_create() if we detect one or more pages are in
> > ZONE_MOVABLE or MIGRATE_CMA would not be a recoverable failure --
> > as it would result in the failure of Guest GUI (or compositor).

Yes, you can't use whatever this driver is while enabling MOVABLE or
CMA in your kernel boot.

> > I think it makes sense to have a generic version of
> > And, since check_and_migrate_movable_pages() is GUP-specific, would
> > it be ok to create a generic version of that (in mm/migrate.c) which can be
> > used by udmabuf and/or other drivers in the future?
> Sorry, I accidentally sent this earlier email before finishing it.
> What I meant to say is since the same situation (inadvertently pinning pages
> in movable) may probably arise in the future with another driver, 

Why?

It was a big mistake to design a uAPI around taking in a FD and
extracting pages from it, we don't have kernel infrastructure for
that, and code liek that does not belong outside the MM at all.

> I think it makes sense to have a generic (non-GUP) version of
> check_and_migrate_movable_pages() available in migration.h that
> drivers can use to ensure that they don't break memory hotunplug
> accidentally.

Definately not.

Either use the VMA and pin_user_pages(), or implement
pin_user_pages_fd() in core code.

Do not open code something wonky in drivers.

Jason