Re: [PATCH v1] x86/mm/pat: (un)track_pfn_copy() fix + improvements

[David Hildenbrand]

On 06.04.25 19:28, Ingo Molnar wrote:
> * David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> > We got a late smatch warning and some additional review feedback.
> >
> > 	smatch warnings:
> > 	mm/memory.c:1428 copy_page_range() error: uninitialized symbol 'pfn'.
>
> > -	if (!(src_vma->vm_flags & VM_PAT))
> > +	if (!(src_vma->vm_flags & VM_PAT)) {
> > +		*pfn = 0;
> >   		return 0;
> > +	}
>
> >   static inline int track_pfn_copy(struct vm_area_struct *dst_vma,
> >   		struct vm_area_struct *src_vma, unsigned long *pfn)
> >   {
> > +	*pfn = 0;
> >   	return 0;
> >   }
>
> That's way too ugly. There's nothing wrong with not touching 'pfn' in
> the error path: in fact it's pretty standard API where output pointers
> may not get set on errors.

We're not concerned about the error path, though.

> If Smatch has a problem with it, Smatch should be fixed, or the false
> positive warning should be worked around by initializing 'pfn' in the
> callers.

We could adjust the documentation of track_pfn_copy, to end up with the
following:

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index e2b705c149454..b50447ef1c921 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -1511,8 +1511,9 @@ static inline void track_pfn_insert(struct vm_area_struct *vma, pgprot_t *prot,
 
 /*
  * track_pfn_copy is called when a VM_PFNMAP VMA is about to get the page
- * tables copied during copy_page_range(). On success, stores the pfn to be
- * passed to untrack_pfn_copy().
+ * tables copied during copy_page_range(). Will store the pfn to be
+ * passed to untrack_pfn_copy() only if there is something to be untracked.
+ * Callers should initialize the pfn to 0.
  */
 static inline int track_pfn_copy(struct vm_area_struct *dst_vma,
                struct vm_area_struct *src_vma, unsigned long *pfn)
@@ -1522,7 +1523,9 @@ static inline int track_pfn_copy(struct vm_area_struct *dst_vma,
 
 /*
  * untrack_pfn_copy is called when a VM_PFNMAP VMA failed to copy during
- * copy_page_range(), but after track_pfn_copy() was already called.
+ * copy_page_range(), but after track_pfn_copy() was already called. Can
+ * be called even if track_pfn_copy() did not actually track anything:
+ * handled internally.
  */
 static inline void untrack_pfn_copy(struct vm_area_struct *dst_vma,
                unsigned long pfn)
diff --git a/mm/memory.c b/mm/memory.c
index 2d8c265fc7d60..1a35165622e1c 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1361,7 +1361,7 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
        struct mm_struct *dst_mm = dst_vma->vm_mm;
        struct mm_struct *src_mm = src_vma->vm_mm;
        struct mmu_notifier_range range;
-       unsigned long next, pfn;
+       unsigned long next, pfn = 0;
        bool is_cow;
        int ret;


--
Cheers,

David / dhildenb