Re: [PATCH 1/3] mm: add functions folio_in_range() and folio_within_vma()

Ryan Roberts
2023-08-02T15:28:28Z

On 28/07/2023 08:09, Yin Fengwei wrote:
> It will be used to check whether the folio is mapped to specific
> VMA and whether the mapping address of folio is in the range.
>
> Also a helper function folio_within_vma() to check whether folio
> is in the range of vma based on folio_in_range().
>
> Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
> ---
> mm/internal.h | 69 +++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 69 insertions(+)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index 5a03bc4782a2..63de32154a48 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -585,6 +585,75 @@ extern long faultin_vma_page_range(struct vm_area_struct *vma,
> bool write, int *locked);
> extern bool mlock_future_ok(struct mm_struct *mm, unsigned long flags,
> unsigned long bytes);
> +
> +/*
> + * Check whether the folio is in specific range
> + *
> + * First, check whether the folio is in the range of vma.
> + * Then, check whether the folio is mapped to the range of [start, end].
> + * In the end, check whether the folio is fully mapped to the range.
> + *
> + * @pte page table pointer will be checked whether the large folio
> + * is fully mapped to. Currently, if mremap in the middle of
> + * large folio, the large folio could be mapped to to different
> + * VMA and address check can't identify this situation.
> + */
> +static inline bool
> +folio_in_range(struct folio *folio, struct vm_area_struct *vma,
> + unsigned long start, unsigned long end, pte_t *pte)
> +{
> + pte_t ptent;
> + unsigned long i, nr = folio_nr_pages(folio);
> + pgoff_t pgoff, addr;
> + unsigned long vma_pglen = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
> +
> + VM_WARN_ON_FOLIO(folio_test_ksm(folio), folio);
> +
> + if (start < vma->vm_start)
> + start = vma->vm_start;
> + if (end > vma->vm_end)
> + end = vma->vm_end;
> +
> + pgoff = folio_pgoff(folio);
> + /* if folio start address is not in vma range */
> + if (pgoff < vma->vm_pgoff || pgoff > vma->vm_pgoff + vma_pglen)
> + return false;

I'm struggling with this logic. What happens for an anonymous folio in a
(private) file mapping? Wouldn't the folio's pgoff be 0? In this case you could
return false incorrectly?

> +
> + addr = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
> + if (addr < start || end - addr < folio_size(folio))
> + return false;
> +
> + /* not necessary to check pte for none large folio */
> + if (!folio_test_large(folio))
> + return true;
> +
> + if (!pte)
> + return false;
> +
> + /* check whether parameter pte is associated with folio */
> + ptent = ptep_get(pte);
> + if (pte_none(ptent) || !pte_present(ptent) ||
> + pte_pfn(ptent) - folio_pfn(folio) >= nr)
> + return false;
> +
> + pte -= pte_pfn(ptent) - folio_pfn(folio);
> + for (i = 0; i < nr; i++, pte++) {
> + ptent = ptep_get(pte);
> +
> + if (pte_none(ptent) || !pte_present(ptent) ||
> + pte_pfn(ptent) - folio_pfn(folio) >= nr)
> + return false;
> + }
> +
> + return true;
> +}
> +
> +static inline bool
> +folio_within_vma(struct folio *folio, struct vm_area_struct *vma, pte_t *pte)
> +{
> + return folio_in_range(folio, vma, vma->vm_start, vma->vm_end, pte);
> +}
> +
> /*
> * mlock_vma_folio() and munlock_vma_folio():
> * should be called with vma's mmap_lock held for read or write,