Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Wed, 17 May 2023 21:20:59 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     Sumit Garg <sumit.garg@linaro.org>,
        Christoph Hellwig <hch@infradead.org>,
        Xiaoming Ding <xiaoming.ding@mediatek.com>,
        Jens Wiklander <jens.wiklander@linaro.org>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>,
        op-tee@lists.trustedfirmware.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org, fei.xu@mediatek.com,
        srv_heupstream@mediatek.com, linux-mm@kvack.org
Subject: FOLL_LONGTERM vs FOLL_EPHEMERAL Re: [PATCH] tee: add FOLL_LONGTERM
 for CMA case when alloc shm
Message-ID: <ZGWnq/dAYELyKpTy@infradead.org>
References: <20230517031856.19660-1-xiaoming.ding@mediatek.com>
 <ZGSDoVKKVqCkbaCB@infradead.org>
 <CAFA6WYO+AvnbuAdWyBAQ8HkLaOno7PXdsPb9SubxrGNvAm4UnQ@mail.gmail.com>
 <ZGSLiiK/JzD5KMd7@infradead.org>
 <CAFA6WYPOMwmrA3J84AHzoD2eAtNkpMxr754qHpc-j6XRkgFFvQ@mail.gmail.com>
 <ZGSgCZrg+RjAbGO1@infradead.org>
 <CAFA6WYO+EpiECFxdVgmd-Ey9jq1Ybt78WupK_bW5+oDcW-soVQ@mail.gmail.com>
 <d4773626-6cf0-c929-c775-a84ac41fd719@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <d4773626-6cf0-c929-c775-a84ac41fd719@redhat.com>
Precedence: bulk

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..