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

Jason Gunthorpe
2021-02-03T14:02:31Z

On Wed, Feb 03, 2021 at 02:43:58PM +0200, Gal Pressman wrote:
> > On Tue, Feb 02, 2021 at 12:05:36PM -0500, Peter Xu wrote:
> >
> >> > Gal, you could also MADV_DONTFORK this range if you are explicitly
> >> > allocating them via special mmap.
> >>
> >> Yeah I wanted to mention this one too but I just forgot when reply: the issue
> >> thread previously pasted smells like some people would like to drop
> >> MADV_DONTFORK, but if it's able to still be applied I don't know why
> >> not..
> >
> > I want to drop the MADV_DONTFORK for dynamic data memory allocated by
> > the application layers (eg with malloc) without knowledge of how they
> > will be used.
> >
> > This case is a buffer internal to the communication system that we
> > know at allocation time how it will be used; so an explicit,
> > deliberate, MADV_DONTFORK is fine
>
> We are referring to libfabric's bounce buffers, correct?
> Libfabric could be considered as the "app" here, it's not clear why these
> buffers should be DONTFORK'd before ibv_reg_mr() but others don't.

I assumed they were internal to the EFA code itself.

> Anyway, it should be simple enough to madvise them after allocation, although I
> think it's part of libfabric's generic code (which isn't necessarily used on
> top of rdma-core).

Ah, so that is a reasonable justification for wanting to fix this in
the kernel..

Lets give Peter some time first.

The other direction to validate this approach is to remove the
MAP_HUGETLB flags and rely on THP instead, and/or mark them as
MAP_SHARED.

I'm not sure generic code should be use using MAP_HUGETLB..

This would be enough to confirm that everything else is working as
expected

Thanks,
Jason