On Thu, 18 May 2023 at 09:51, Christoph Hellwig hch@infradead.org wrote:
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..
It does look like a better approach to me given the very nature of user space pages.
-Sumit