linux-kernel - Re: [PATCH v1] x86/mm/pat: (un)track_pfn

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Message-ID: <630caa8e-2ee2-4895-9e4e-8bf2fa079100@redhat.com>
Date: Mon, 7 Apr 2025 09:23:06 +0200
From: David Hildenbrand <david@...hat.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
 kernel test robot <lkp@...el.com>, Dan Carpenter <error27@...il.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski
 <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Rik van Riel <riel@...riel.com>,
 "H. Peter Anvin" <hpa@...or.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v1] x86/mm/pat: (un)track_pfn_copy() fix + improvements

On 06.04.25 19:28, Ingo Molnar wrote:
> 
> * David Hildenbrand <david@...hat.com> 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