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

[Nick Piggin <npiggin@suse.de>]

On Mon, Oct 09, 2006 at 08:50:14PM +1000, Benjamin Herrenschmidt wrote:
> 
> > The truncate logic can't be duplicated because it works on struct pages.
> > 
> > What sounds best, if you use nopfn, is to do your own internal
> > synchronisation against your unmap call. Obviously you can't because you
> > have no ->nopfn_done call with which to drop locks ;)
> > 
> > So, hmm yes I have a good idea for how fault() could take over ->nopfn as
> > well: just return NULL, set the fault type to VM_FAULT_MINOR, and have
> > the ->fault handler install the pte. It will require a new helper along
> > the lines of vm_insert_page.
> > 
> > I'll code that up in my next patchset.
> 
> Which is exactly what I was proposing in my other mail :)
> 
> Read it, you'll understnad my point about the truncate logic... I
> sometimes want to return struct page (when the mapping is pointing to
> backup memory) or map it directly to hardware.
> 
> In the later case, with an appropriate helper, I can definitely do my
> own locking. In the former case, I return struct page's and need the
> truncate logic.

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.
And going the pageless route means that you must disallow MAP_PRIVATE
PROT_WRITE mappings, I trust that isn't a problem for you?

I guess the helper looks something like the following...

--

Index: linux-2.6/include/linux/mm.h
===================================================================
--- linux-2.6.orig/include/linux/mm.h
+++ linux-2.6/include/linux/mm.h
@@ -1105,6 +1105,7 @@ unsigned long vmalloc_to_pfn(void *addr)
 int remap_pfn_range(struct vm_area_struct *, unsigned long addr,
 			unsigned long pfn, unsigned long size, pgprot_t);
 int vm_insert_page(struct vm_area_struct *, unsigned long addr, struct page *);
+int vm_insert_pfn(struct vm_area_struct *, unsigned long addr, unsigned long pfn);
 
 struct page *follow_page(struct vm_area_struct *, unsigned long address,
 			unsigned int foll_flags);
Index: linux-2.6/mm/memory.c
===================================================================
--- linux-2.6.orig/mm/memory.c
+++ linux-2.6/mm/memory.c
@@ -1267,6 +1267,44 @@ int vm_insert_page(struct vm_area_struct
 }
 EXPORT_SYMBOL(vm_insert_page);
 
+/**
+ * vm_insert_pfn - insert single pfn into user vma
+ * @vma: user vma to map to
+ * @addr: target user address of this page
+ * @pfn: source kernel pfn
+ *
+ * Similar to vm_inert_page, this allows drivers to insert individual pages
+ * they've allocated into a user vma. Same comments apply
+ */
+int vm_insert_pfn(struct vm_area_struct *vma, unsigned long addr, unsigned long pfn)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	int retval;
+	pte_t *pte;
+	spinlock_t *ptl;
+
+	BUG_ON(is_cow_mapping(vma->vm_flags));
+
+	retval = -ENOMEM;
+	pte = get_locked_pte(mm, addr, &ptl);
+	if (!pte)
+		goto out;
+	retval = -EBUSY;
+	if (!pte_none(*pte))
+		goto out_unlock;
+
+	/* 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);
+
 /*
  * maps a range of physical memory into the requested pages. the old
  * mappings are removed. any references to nonexistent pages results

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