Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned

John Hubbard
2020-09-29T00:18:53Z

On 9/28/20 4:57 PM, Jason Gunthorpe wrote:
> On Mon, Sep 28, 2020 at 12:29:55PM -0700, Linus Torvalds wrote:
...
> I think this is really hard to use and ugly. My thinking has been to
> just stick:
>
> if (flags & FOLL_LONGTERM)
> flags |= FOLL_FORCE | FOLL_WRITE
>
> In pin_user_pages(). It would make the driver API cleaner. If we can

+1, yes. The other choices so far are, as you say, really difficult to figure
out.

> do a bit better somehow by not COW'ing for certain VMA's as you
> explained then all the better, but not my primary goal..
>
> Basically, I think if a driver is using FOLL_LONGTERM | FOLL_PIN we
> should guarentee that driver a consistent MM and take the gup_fast
> performance hit to do it.
>
> AFAICT the giant wack of other cases not using FOLL_LONGTERM really
> shouldn't care about read-decoherence. For those cases the user should
> really not be racing write's with data under read-only pin, and the
> new COW logic looks like it solves the other issues with this.

I hope this doesn't kill the seqcount() idea, though. That was my favorite
part of the discussion, because it neatly separates out the two racing domains
(fork, gup/pup) and allows easy reasoning about them--without really impacting
performance.

Truly elegant. We should go there.

>
> I know Jann/John have been careful to not have special behaviors for
> the DMA case, but I think it makes sense here. It is actually different.
>

I think that makes sense. Everyone knew that DMA/FOLL_LONGTERM call sites
were at least potentially special, despite the spirited debates in at least
two conferences about the meaning and implications of "long term". :)

And here we are seeing an example of such a special case, which I think is
natural enough.

thanks,
--
John Hubbard
NVIDIA