Re: [PATCH v2 bpf-next 04/20] mm: Expose vmap_pages_range() to the rest of the kernel.

[Christoph Hellwig <hch@infradead.org>]

On Wed, Feb 14, 2024 at 12:53:42PM -0800, Alexei Starovoitov wrote:
> On Wed, Feb 14, 2024 at 12:36 AM Christoph Hellwig <hch@infradead.org> wrote:
> >
> > NAK.  Please
> 
> What is the alternative?
> Remember, maintainers cannot tell developers "go away".
> They must suggest a different path.

That criteria is something you've made up.   Telling that something
is not ok is the most important job of not just maintainers but all
developers.  Maybe start with a description of the problem you're
solving and why you think it matters and needs different APIs.

> . get_vm_area - external
> . free_vm_area - EXPORT_SYMBOL_GPL
> . vunmap_range - external
> . vmalloc_to_page - EXPORT_SYMBOL
> . apply_to_page_range - EXPORT_SYMBOL_GPL
> 
> and the last one is pretty much equivalent to vmap_pages_range,
> hence I'm surprised by push back to make vmap_pages_range available to bpf.

And the last we've been trying to get rid of by ages because we don't
want random modules to 

> > > For example, there is the public ioremap_page_range(), which is used
> > > to map device memory into addressable kernel space.
> >
> > It's not really public.  It's a helper for the ioremap implementation
> > which really should not be arch specific to start with and are in
> > the process of beeing consolidatd into common code.
> 
> Any link to such consolidation of ioremap ? I couldn't find one.

Second hit on google:

https://lore.kernel.org/lkml/20230609075528.9390-1-bhe@redhat.com/T/

> I surely don't want bpf_arena to cause headaches to mm folks.
> 
> Anyway, ioremap_page_range() was just an example.
> I could have used vmap() as an equivalent example.
> vmap is EXPORT_SYMBOL, btw.

vmap is a good well defined API.  vmap_pages_range is not.

> What bpf_arena needs is pretty much vmap(), but instead of
> allocating all pages in advance, allocate them and insert on demand.

So propose an API that does that instead of exposing random low-level
details.