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

[Christoph Hellwig]

On Wed, May 17, 2023 at 08:23:33PM +0200, David Hildenbrand wrote:
> In general: if user space controls it -> possibly forever -> long-term. Even
> if in most cases it's a short delay: there is no trusting on user space.
> 
> For example, iouring fixed buffers keep pages pinned until user space
> decides to unregistered the buffers -> long-term.
> 
> Short-term is, for example, something like O_DIRECT where we pin -> DMA ->
> unpin in essentially one operation.

Btw, one thing that's been on my mind is that I think we got the
polarity on FOLL_LONGTERM wrong.  Instead of opting into the long term
behavior it really should be the default, with a FOLL_EPHEMERAL flag
to opt out of it.  And every users of this flag is required to have
a comment explaining the life time rules for the pin..