Messages in this thread | | | Subject | Re: [PATCH 1/4] mm: Trial do_wp_page() simplification | From | John Hubbard <> | Date | Fri, 18 Sep 2020 14:06:23 -0700 |
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On 9/18/20 1:40 PM, Peter Xu wrote: > On Fri, Sep 18, 2020 at 02:32:40PM -0300, Jason Gunthorpe wrote: >> On Fri, Sep 18, 2020 at 12:40:32PM -0400, Peter Xu wrote: >> >>> Firstly in the draft patch mm->has_pinned is introduced and it's written to 1 >>> as long as FOLL_GUP is called once. It's never reset after set. >> >> Worth thinking about also adding FOLL_LONGTERM here, at last as long >> as it is not a counter. That further limits the impact. > > But theoritically we should also trigger COW here for pages even with PIN && > !LONGTERM, am I right? Assuming that FOLL_PIN is already a corner case. >
This note, plus Linus' comment about "I'm a normal process, I've never done any special rdma page pinning", has me a little worried. Because page_maybe_dma_pinned() is counting both short- and long-term pins, actually. And that includes O_DIRECT callers.
O_DIRECT pins are short-term, and RDMA systems are long-term (and should be setting FOLL_LONGTERM). But there's no way right now to discern between them, once the initial pin_user_pages*() call is complete. All we can do today is to count the number of FOLL_PIN calls, not the number of FOLL_PIN | FOLL_LONGTERM calls.
The reason it's that way, is that writeback and such can experience problems regardless of the duration of the pin. There are ideas about how to deal with the pins, and the filesystem (layout leases...) but still disagreement, which is why there's basically no page_maybe_dma_pinned() callers yet.
Although I think we're getting closer to using it. There was a recent attempt at using this stuff, from Chris Wilson. [1]
[1] https://lore.kernel.org/intel-gfx/20200624191417.16735-1-chris%40chris-wilson.co.uk/
thanks, -- John Hubbard NVIDIA
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