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

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Date:   Wed, 17 May 2023 02:36:09 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Sumit Garg <sumit.garg@...aro.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Xiaoming Ding <xiaoming.ding@...iatek.com>,
        Jens Wiklander <jens.wiklander@...aro.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        op-tee@...ts.trustedfirmware.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, fei.xu@...iatek.com,
        srv_heupstream@...iatek.com, linux-mm@...ck.org
Subject: Re: [PATCH] tee: add FOLL_LONGTERM for CMA case when alloc shm

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.