Re: [PATCH] tee: add FOLL_LONGTERM for CMA case when alloc shm

[Christoph Hellwig]

On Wed, May 17, 2023 at 02:56:13PM +0530, Sumit Garg wrote:
> Do you mean a pinned user-space page can be paged out automatically?

No, pinned pages can't be paged out.

But a short term pin implies it will be release after a short delay,
and it is feasible for wait for the pin to go away.

For a long term pin waiting is not an option, and anyone wanting to
do something with the pinned page that requires it to not be pinned
must simply give up.

> Just FYI, the underlying use-case for TEE registered shared memory is
> that the references to pinned pages are provided to TEE implementation
> to operate upon. This can happen over multiple syscalls and we want
> the pinned pages to be always in RAM as otherwise the physical
> addresses may change if they are paged out in between.

That's a very use clear case for a long term pin.