linux-kernel - [PATCH v2] x86/mm/pat: (un)track_pfn

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Message-ID: <20250408085950.976103-1-david@redhat.com>
Date: Tue,  8 Apr 2025 10:59:50 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org,
	x86@...nel.org,
	David Hildenbrand <david@...hat.com>,
	kernel test robot <lkp@...el.com>,
	Dan Carpenter <error27@...il.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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: [PATCH v2] x86/mm/pat: (un)track_pfn_copy() fix + doc improvements

We got a late smatch warning and some additional review feedback.

	smatch warnings:
	mm/memory.c:1428 copy_page_range() error: uninitialized symbol 'pfn'.

We actually use the pfn only when it is properly initialized; however,
we may pass an uninitialized value to a function -- although it will not
use it that likely still is UB in C.

So let's just fix it by always initializing pfn in the caller of
track_pfn_copy(), and improving the documentation of track_pfn_copy().

While at it, clarify the doc of untrack_pfn_copy(), that internal checks
make sure if we actually have to untrack anything.

Fixes: dc84bc2aba85 ("x86/mm/pat: Fix VM_PAT handling when fork() fails in copy_page_range()")
Reported-by: kernel test robot <lkp@...el.com>
Reported-by: Dan Carpenter <error27@...il.com>
Closes: https://lore.kernel.org/r/202503270941.IFILyNCX-lkp@intel.com/
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Rik van Riel <riel@...riel.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: David Hildenbrand <david@...hat.com>
---

v1 -> v2:
* Adjust the doc instead of initializing the pfn whenever returning 0
* Decided to keep Lorenzo's RB :)
* Retested

---
 include/linux/pgtable.h | 9 ++++++---
 mm/memory.c             | 2 +-
 2 files changed, 7 insertions(+), 4 deletions(-)

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;
 
-- 
2.48.1