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

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Date:   Wed, 17 May 2023 20:23:33 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Sumit Garg <sumit.garg@...aro.org>,
        Christoph Hellwig <hch@...radead.org>,
        Xiaoming Ding <xiaoming.ding@...iatek.com>
Cc:     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 17.05.23 12:19, Sumit Garg wrote:
> On Wed, 17 May 2023 at 15:06, Christoph Hellwig <hch@...radead.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