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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, -- John Hubbard NVIDIA