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

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Message-Id: <1160393579.10229.24.camel@localhost.localdomain>
Date:	Mon, 09 Oct 2006 21:32:59 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Andrew Morton <akpm@...l.org>,
	Linux Memory Management <linux-mm@...ck.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [patch 3/3] mm: fault handler to replace nopage and populate

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.


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