[RFC] Changing VM_PFNMAP assumptions and rules

["Jared Hulbert" <jaredeh@gmail.com>]

Per conversations regarding XIP from the vm/fs mini-summit a couple
months back I've got a patch to air out.

The basic problem is that the assumptions about PFN mappings stemming
from the rules of remap_pfn_range() aren't always valid.  For example:
what stops one from using vm_insert_pfn() to map PFN's into a vma in
an arbitrary order?  Nothing.  Yet those PFN's cause problems in two
ways.

First, vm_normal_page() won't return NULL.  My answer to this is to
simply check if pfn_valid()  if it isn't then we've got a proper PFN
that can only be a PFN.  If you do have a valid PFN then you are (A) a
'cow'ed' PFN that is now a real page or (B) you are a real page
pretending to be a PFN only.  The thing that makes me nervous is that
my hack doesn't let that page pretend to be a PFN.  I can't figure out
why a page would need/want to pretend to be a PFN so I don't see
anything wrong with this, but maybe somebody does.

Second, there are a few random BUG_ON() that don't seem to serve any
purpose other than to punish the PFN's that don't abide by
remap_pfn_range() rules.  I just get rid of them.  The problem is I
don't really understand why they are there in the first place so for
all I know I'm horribly breaking spufs or something.

Okay so I haven't tried this out on 2.6.24-rc1 yet, but the same basic
idea worked on 2.6.23 and older.  I just wanted to get feedback on
this approach.  I don't know the vm all that well so I want to make
sure I'm not doing something really stupid that breaks a bunch of code
paths I don't use.


diff --git a/mm/memory.c b/mm/memory.c
index 9791e47..fb962d0 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -366,29 +366,19 @@ static inline int is_cow_mapping(unsigned int flags)
  * NOTE! Some mappings do not have "struct pages". A raw PFN mapping
  * will have each page table entry just pointing to a raw page frame
  * number, and as far as the VM layer is concerned, those do not have
- * pages associated with them - even if the PFN might point to memory
- * that otherwise is perfectly fine and has a "struct page".
+ * pages associated with them.
  *
- * The way we recognize those mappings is through the rules set up
- * by "remap_pfn_range()": the vma will have the VM_PFNMAP bit set,
- * and the vm_pgoff will point to the first PFN mapped: thus every
- * page that is a raw mapping will always honor the rule
- *
- *	pfn_of_page == vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT)
- *
- * and if that isn't true, the page has been COW'ed (in which case it
- * _does_ have a "struct page" associated with it even if it is in a
- * VM_PFNMAP range).
+ * The old "remap_pfn_range()" rules don't work for all applications.
+ * Each "page" in a PFN mapping either has a page struct backing it
+ * or it doesn't.  If it does then treat it like the page it is, if
+ * if it doesn't then it is not a normal page so just return NULL.
  */
 struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long
addr, pte_t pte)
 {
 	unsigned long pfn = pte_pfn(pte);

 	if (unlikely(vma->vm_flags & VM_PFNMAP)) {
-		unsigned long off = (addr - vma->vm_start) >> PAGE_SHIFT;
-		if (pfn == vma->vm_pgoff + off)
-			return NULL;
-		if (!is_cow_mapping(vma->vm_flags))
+		if (!pfn_valid(pfn))
 			return NULL;
 	}

@@ -1212,7 +1202,6 @@ int vm_insert_pfn(struct vm_area_struct *vma,
unsigned long addr,
 	spinlock_t *ptl;

 	BUG_ON(!(vma->vm_flags & VM_PFNMAP));
-	BUG_ON(is_cow_mapping(vma->vm_flags));

 	retval = -ENOMEM;
 	pte = get_locked_pte(mm, addr, &ptl);
@@ -2216,8 +2205,6 @@ static int __do_fault(struct mm_struct *mm,
struct vm_area_struct *vma,
 	vmf.flags = flags;
 	vmf.page = NULL;

-	BUG_ON(vma->vm_flags & VM_PFNMAP);
-
 	if (likely(vma->vm_ops->fault)) {
 		ret = vma->vm_ops->fault(vma, &vmf);
 		if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))

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