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

On Wed, 17 May 2023 at 15:06, Christoph Hellwig [off-list ref] wrote:

On Wed, May 17, 2023 at 02:56:13PM +0530, Sumit Garg wrote:

quoted

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.

Okay, I see. I would be interested to know the ranges for that short
delay. I guess it may depend on how much memory pressure there is...

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.

quoted

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.

...however, thanks for the insights.

@Xiaoming,

Please use the following fixes tag for the v2 along with extending the
commit description regarding the reliability provided by the long term
flag.

Fixes: 033ddf12bcf5 ("tee: add register user memory")

-Sumit

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel