Re: [patch 3/3] mm: fault handler to replace nopage and populate

Benjamin Herrenschmidt
2006-10-09T11:33:26Z

On Mon, 2006-10-09 at 13:19 +0200, Nick Piggin wrote:
> On Mon, Oct 09, 2006 at 09:10:13PM +1000, Benjamin Herrenschmidt wrote:
> >
> > > Yep, I see. You just need to be careful about the PFNMAP logic, so
> > > the VM knows whether the pte is backed by a struct page or not.
> >
> > I still need to properly get my head around that one. I can't easily
> > change the VMA during the "switch" but I can tweak the flags on the
> > first nopage after one...
>
> You'll want to clear VM_PFNMAP after unmapping all pages from it, before
> switching to struct page backing.

Which means having a list of all vma's ... I suppose I can look at the
truncate code to do that race free but I was hoping I could avoid it
(that's the whole point of using unmap_mapping_range() in fact).

> > > And going the pageless route means that you must disallow MAP_PRIVATE
> > > PROT_WRITE mappings, I trust that isn't a problem for you?
> >
> > Should not but I need to look more closely.
>
> If you do need to, then if your pfns are contiguous in virtual memory,
> and you can spare vm_pgoff, then you can use remap_pfn_range's method
> of setting vm_pgoff to the first pfn.

Yup. I got that bit.

> I can add a bit of sanity checking for that as well.
>
> > > + /* Ok, finally just insert the thing.. */
> > > + set_pte_at(mm, addr, pte, pfn_pte(pfn, vma->vm_page_prot));
> > > +
> > > + vma->vm_flags |= VM_PFNMAP;
> > > + retval = 0;
> > > +out_unlock:
> > > + pte_unmap_unlock(pte, ptl);
> > > +out:
> > > + return retval;
> > > +}
> > > +EXPORT_SYMBOL(vm_insert_pfn);
> >
> > It also needs update_mmu_cache() I suppose.
>
> Hmm, but it might not be called from a pagefault. Can we get away
> with not calling it? Or is it required by some architectures?

I think some architectures might be upset if it's not called...

Ben.