Re: [PATCH 1/4] mm: Trial do_wp_page() simplification

Jan Kara
2020-09-21T08:35:09Z

On Fri 18-09-20 21:01:53, Jason Gunthorpe wrote:
> On Fri, Sep 18, 2020 at 02:06:23PM -0700, John Hubbard wrote:
> > 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.
>
> My thinking is to hit this issue you have to already be doing
> FOLL_LONGTERM, and if some driver hasn't been properly marked and
> regresses, the fix is to mark it.
>
> Remember, this use case requires the pin to extend after a system
> call, past another fork() system call, and still have data-coherence.
>
> IMHO that can only happen in the FOLL_LONGTERM case as it inhernetly
> means the lifetime of the pin is being controlled by userspace, not by
> the kernel. Otherwise userspace could not cause new DMA touches after
> fork.

I agree that the new aggressive COW behavior is probably causing issues
only for FOLL_LONGTERM users. That being said it would be nice if even
ordinary threaded FOLL_PIN users would not have to be that careful about
fork(2) and possible data loss due to COW - we had certainly reports of
O_DIRECT IO loosing data due to fork(2) and COW exactly because it is very
subtle how it behaves... But as I wrote above this is not urgent since that
problematic behavior exists since the beginning of O_DIRECT IO in Linux.

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR