Date: Mon, 9 Oct 2006 13:19:06 +0200
From: Nick Piggin <npiggin@suse.de>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Andrew Morton <akpm@osdl.org>,
       Linux Memory Management <linux-mm@kvack.org>,
       Linux Kernel <linux-kernel@vger.kernel.org>
Subject: Re: [patch 3/3] mm: fault handler to replace nopage and populate
Message-ID: <20061009111906.GA26824@wotan.suse.de>
References: <20061007105758.14024.70048.sendpatchset@linux.site> <20061007105853.14024.95383.sendpatchset@linux.site> <20061007134407.6aa4dd26.akpm@osdl.org> <1160351174.14601.3.camel@localhost.localdomain> <20061009102635.GC3487@wotan.suse.de> <1160391014.10229.16.camel@localhost.localdomain> <20061009110007.GA3592@wotan.suse.de> <1160392214.10229.19.camel@localhost.localdomain>
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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.

> 
> > 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.

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?
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