Re: FOLL_LONGTERM vs FOLL_EPHEMERAL Re: [PATCH] tee: add FOLL_LONGTERM for CMA case when alloc shm
From: Sumit Garg <hidden>
Date: 2023-05-18 06:08:37
Also in:
linux-mediatek, linux-mm, lkml, op-tee
On Thu, 18 May 2023 at 09:51, Christoph Hellwig [off-list ref] wrote:
On Wed, May 17, 2023 at 08:23:33PM +0200, David Hildenbrand wrote:quoted
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..
It does look like a better approach to me given the very nature of user space pages. -Sumit _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel