Re: [PATCH v2 bpf-next 3/3] mm: Introduce VM_SPARSE kind and vm_area_[un]map_pages().

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Tue, Feb 27, 2024 at 9:59 AM Christoph Hellwig <hch@infradead.org> wrote:
>
> > privately-managed pages into a sparse vm area with the following steps:
> >
> >   area = get_vm_area(area_size, VM_SPARSE);  // at bpf prog verification time
> >   vm_area_map_pages(area, kaddr, 1, page);   // on demand
> >                     // it will return an error if kaddr is out of range
> >   vm_area_unmap_pages(area, kaddr, 1);
> >   free_vm_area(area);                        // after bpf prog is unloaded
>
> I'm still wondering if this should just use an opaque cookie instead
> of exposing the vm_area.  But otherwise this mostly looks fine to me.

What would it look like with a cookie?
A static inline wrapper around get_vm_area() that returns area->addr ?
And the start address of vmap range will be such a cookie?

Then vm_area_map_pages() will be doing find_vm_area() for kaddr
to check that vm_area->flag & VM_SPARSE ?
That's fine,
but what would be an equivalent of void free_vm_area(struct vm_struct *area) ?
Another static inline wrapper similar to remove_vm_area()
that also does kfree(area); ?

Fine by me, but api isn't user friendly with such obfuscation.

I guess I don't understand the motivation to hide 'struct vm_struct *'.

> > +     if (addr < (unsigned long)area->addr || (void *)end > area->addr + area->size)
> > +             return -ERANGE;
>
> This check is duplicated so many times that it really begs for a helper.

ok. will do.

> > +int vm_area_unmap_pages(struct vm_struct *area, unsigned long addr, unsigned int count)
> > +{
> > +     unsigned long size = ((unsigned long)count) * PAGE_SIZE;
> > +     unsigned long end = addr + size;
> > +
> > +     if (WARN_ON_ONCE(!(area->flags & VM_SPARSE)))
> > +             return -EINVAL;
> > +     if (addr < (unsigned long)area->addr || (void *)end > area->addr + area->size)
> > +             return -ERANGE;
> > +
> > +     vunmap_range(addr, end);
> > +     return 0;
>
> Does it make much sense to have an error return here vs just debug
> checks?  It's not like the caller can do much if it violates these
> basic invariants.

Ok. Will switch to void return.

Will reduce commit line logs to 75 chars in all patches as suggested.

re: VM_GRANT_TABLE or VM_XEN_GRANT_TABLE suggestion for patch 2.

I'm not sure it fits, since only one of get_vm_area() in xen code
is a grant table related. The other one is for xenbus that
creates a shared memory ring between domains.
So I'm planning to keep it as VM_XEN in the next revision unless
folks come up with a better name.

Thanks for the reviews.