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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 > Reported-by: Dan Carpenter > Closes: https://lore.kernel.org/r/202503270941.IFILyNCX-lkp@intel.com/ > Reviewed-by: Lorenzo Stoakes > Cc: Andrew Morton > Cc: Lorenzo Stoakes > Cc: Dave Hansen > Cc: Andy Lutomirski > Cc: Peter Zijlstra > Cc: Thomas Gleixner > Cc: Ingo Molnar > Cc: Borislav Petkov > Cc: Rik van Riel > Cc: "H. Peter Anvin" > Cc: Linus Torvalds > Signed-off-by: David Hildenbrand > --- > > 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 >