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

Peter Xu
2021-02-02T17:09:36Z

On Tue, Feb 02, 2021 at 12:44:20PM -0400, Jason Gunthorpe wrote:
> On Tue, Feb 02, 2021 at 11:31:27AM -0500, Peter Xu wrote:
> > On Tue, Feb 02, 2021 at 04:40:33PM +0200, Gal Pressman wrote:
> > > Hi Peter & Jason,
> >
> > Hi, Gal, Jason,
> >
> > >
> > > It seems the hugetlb part was overlooked?
> > > We're testing if the RDMA fork MADV_DONTFORK stuff can be removed on appropriate
> > > kernels, but our tests still fail due to lacking explicit huge pages support [1].
> >
> > I didn't think it high priority only because I think most hugetlbfs users
> > should be using it shared, but maybe I'm wrong.. Then it got lost indeed.
>
> It turns out people are doing this:
>
> mmap(NULL, SEND_BUFF_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB, -1, 0)
>
> Which makes some sense...

Yes, thanks Jason. Though my understanding is that hugetlb pages are normally
reserved in production, used with careful pre-provisioning on which app would
consume how much (either 2M or 1G). Such an app (especially if it forks
randomly) could actually easily exaust the huge page pool.

>
> 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.. It
should still be better than depending on the coming patch imho - it marks the
region as "not necessary for the fork" then we skip the whole hugetlbfs chunk.
It should at least be more efficient if applicable.

Thanks,

--
Peter Xu