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

Jason Gunthorpe
2020-09-28T12:49:41Z

On Sun, Sep 27, 2020 at 11:45:30AM -0700, Linus Torvalds wrote:
> On Sun, Sep 27, 2020 at 11:16 AM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > Btw, I'm not convinced about the whole "turn the pte read-only and
> > then back". If the fork races with another thread doing a pinning
> > fast-GUP on another CPU, there are memory ordering issues etc too.
> > That's not necessarily visible on x86 (the "turn read-only being a
> > locked op will force serialization), but it all looks dodgy as heck.

Oh. Yes, looking again the atomics in the final arrangement of
copy_present_page() aren't going to be strong enough to order this.

Sorry for missing, wasn't able to look very deeply during the weekend.

Not seeing an obvious option besides adding a smp_mb() before
page_maybe_dma_pinned() as Peter once suggested.

> .. looking at it more, I also think it could possibly lose the dirty
> bit for the case where another CPU did a HW dirty/accessed bit update
> in between the original read of the pte, and then us writing back the
> writable pte again.

Ah, I see:

set_pte_at(src_mm, addr, src_pte, pte);

wants to be some arch specific single bit ptep_clear_wrprotect()..

Thanks,
Jason