Re: [PATCH v2] x86/mm/pat: (un)track_pfn_copy() fix + doc improvements

[Lorenzo Stoakes]

On Tue, Apr 08, 2025 at 10:59:50AM +0200, David Hildenbrand 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'.
>
> 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@xxxxxxxxx>
> Reported-by: Dan Carpenter <error27@xxxxxxxxx>
> Closes: https://lore.kernel.org/r/202503270941.IFILyNCX-lkp@xxxxxxxxx/
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>
> v1 -> v2:
> * Adjust the doc instead of initializing the pfn whenever returning 0
> * Decided to keep Lorenzo's RB :)

All good with me :) the patch in this form is fine!

> * 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
>