On 5/18/23 06:56, David Hildenbrand wrote:
On 18.05.23 08:08, Sumit Garg wrote:
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..
I see maybe 10 or 20 call sites today. So it is definitely feasible to add documentation at each, explaining the why it wants a long term pin.
It does look like a better approach to me given the very nature of user space pages.
Yeah, there is a lot of historical baggage. For example, FOLL_GET should be inaccessible to kernel modules completely at one point, to be only used by selected core-mm pieces.
Yes. When I first mass-converted call sites from gup to pup, I just preserved FOLL_GET behavior in order to keep from changing too much at once. But I agree that that it would be nice to make FOLL_GET an mm internal-only flag like FOLL_PIN.
Maybe we should even disallow passing in FOLL_LONGTERM as a flag and only provide functions like pin_user_pages() vs. pin_user_pages_longterm(). Then, discussions about conditional flag-setting are no more :)
... or even use pin_user_pages_shortterm() vs. pin_user_pages() ... to make the default be longterm.
Yes, it is true that having most gup flags be internal to mm does tend to avoid some bugs. But it's also a lot of churn. I'm still on the fence as to whether it's really a good move to do this for FOLL_LONGTERM or not. But it's really easy to push me off of fences. :)
thanks,