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Date:   Wed, 17 May 2023 20:23:33 +0200
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Subject: Re: [PATCH] tee: add FOLL_LONGTERM for CMA case when alloc shm
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To:     Sumit Garg <sumit.garg@linaro.org>,
        Christoph Hellwig <hch@infradead.org>,
        Xiaoming Ding <xiaoming.ding@mediatek.com>
Cc:     Jens Wiklander <jens.wiklander@linaro.org>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>,
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        linux-mediatek@lists.infradead.org, fei.xu@mediatek.com,
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
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On 17.05.23 12:19, Sumit Garg wrote:
> On Wed, 17 May 2023 at 15:06, Christoph Hellwig <hch@infradead.org> wrote:
>>
>> 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.
> 
> 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...
> 

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.

-- 
Thanks,

David / dhildenb