linux-kernel - FOLL_LONGTERM vs FOLL_EPHEMERAL Re: [PATCH] tee: add FOLL

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Date:   Wed, 17 May 2023 21:20:59 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Sumit Garg <sumit.garg@...aro.org>,
        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: FOLL_LONGTERM vs FOLL_EPHEMERAL Re: [PATCH] tee: add FOLL_LONGTERM
 for CMA case when alloc shm

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..