Re: [PATCH v2 16/19] mm: Remove follow_pte()

[Yu Zhao]

On Mon, Aug 26, 2024 at 2:44 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> follow_pte() users have been converted to follow_pfnmap*().  Remove the
> API.
>
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
>  include/linux/mm.h |  2 --
>  mm/memory.c        | 73 ----------------------------------------------
>  2 files changed, 75 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 161d496bfd18..b31d4bdd65ad 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2368,8 +2368,6 @@ void free_pgd_range(struct mmu_gather *tlb, unsigned long addr,
>                 unsigned long end, unsigned long floor, unsigned long ceiling);
>  int
>  copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma);
> -int follow_pte(struct vm_area_struct *vma, unsigned long address,
> -              pte_t **ptepp, spinlock_t **ptlp);
>  int generic_access_phys(struct vm_area_struct *vma, unsigned long addr,
>                         void *buf, int len, int write);
>
> diff --git a/mm/memory.c b/mm/memory.c
> index b5d07f493d5d..288f81a8698e 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -6100,79 +6100,6 @@ int __pmd_alloc(struct mm_struct *mm, pud_t *pud, unsigned long address)
>  }
>  #endif /* __PAGETABLE_PMD_FOLDED */
>
> -/**
> - * follow_pte - look up PTE at a user virtual address
> - * @vma: the memory mapping
> - * @address: user virtual address
> - * @ptepp: location to store found PTE
> - * @ptlp: location to store the lock for the PTE
> - *
> - * On a successful return, the pointer to the PTE is stored in @ptepp;
> - * the corresponding lock is taken and its location is stored in @ptlp.
> - *
> - * The contents of the PTE are only stable until @ptlp is released using
> - * pte_unmap_unlock(). This function will fail if the PTE is non-present.
> - * Present PTEs may include PTEs that map refcounted pages, such as
> - * anonymous folios in COW mappings.
> - *
> - * Callers must be careful when relying on PTE content after
> - * pte_unmap_unlock(). Especially if the PTE maps a refcounted page,
> - * callers must protect against invalidation with MMU notifiers; otherwise
> - * access to the PFN at a later point in time can trigger use-after-free.
> - *
> - * Only IO mappings and raw PFN mappings are allowed.  The mmap semaphore
> - * should be taken for read.
> - *
> - * This function must not be used to modify PTE content.
> - *
> - * Return: zero on success, -ve otherwise.
> - */
> -int follow_pte(struct vm_area_struct *vma, unsigned long address,
> -              pte_t **ptepp, spinlock_t **ptlp)
> -{
> -       struct mm_struct *mm = vma->vm_mm;
> -       pgd_t *pgd;
> -       p4d_t *p4d;
> -       pud_t *pud;
> -       pmd_t *pmd;
> -       pte_t *ptep;
> -
> -       mmap_assert_locked(mm);
> -       if (unlikely(address < vma->vm_start || address >= vma->vm_end))
> -               goto out;
> -
> -       if (!(vma->vm_flags & (VM_IO | VM_PFNMAP)))
> -               goto out;
> -
> -       pgd = pgd_offset(mm, address);
> -       if (pgd_none(*pgd) || unlikely(pgd_bad(*pgd)))
> -               goto out;
> -
> -       p4d = p4d_offset(pgd, address);
> -       if (p4d_none(*p4d) || unlikely(p4d_bad(*p4d)))
> -               goto out;
> -
> -       pud = pud_offset(p4d, address);
> -       if (pud_none(*pud) || unlikely(pud_bad(*pud)))
> -               goto out;
> -
> -       pmd = pmd_offset(pud, address);
> -       VM_BUG_ON(pmd_trans_huge(*pmd));
> -
> -       ptep = pte_offset_map_lock(mm, pmd, address, ptlp);
> -       if (!ptep)
> -               goto out;
> -       if (!pte_present(ptep_get(ptep)))
> -               goto unlock;
> -       *ptepp = ptep;
> -       return 0;
> -unlock:
> -       pte_unmap_unlock(ptep, *ptlp);
> -out:
> -       return -EINVAL;
> -}
> -EXPORT_SYMBOL_GPL(follow_pte);

I ran into build errors with this -- removing exported symbols breaks
ABI, so I think we should make follow_pte() as a wrapper of its new
equivalent, if that's possible?