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

[Ryan Roberts]

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@xxxxxxxxx>
> ---
>  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);
> +

Hi Yin,

I wanted to take a step back and consolidate my concerns about this function. I
should say that my understanding of the pgoff and index stuff is shaky and I
could therefore be wrong about some of this; if this is the case, then sorry for
the noise! But something about this function doesn't smell right to me, so I
figure its better to raise it...

> +/*
> + * Check whether the folio is in specific range

What exactly is the function trying to do? I *think* the intention is to figure
out if a folio is fully and contiguously mapped within a range of virtual
addresses belonging to a specific virtual address space? And I assume you want
the answer to be precise? I'm assuming 'yes' for the below.

I think the only way to do this is to actually check each PTE. And that causes a
problem, because a folio can straddle multiple PTE tables, which causes PTL
locking issues, and means having just a *pte pointer is insufficient if we need
to traverse multiple pte tables. So perhaps you need to allow for a false
negative in the case that the folio is not contained within a single pte table.

> + *
> + * 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)

The prototype looks odd to me; Fundamentally it seems to me that you need the
folio, start and end virtual addresses (the range you want to check that it is
in), the pte pointer and the virtual address that the pte represents. That
virtual address allows you to figure out the offset between the pa and va. Then
you can look at all the ptes to figure out if they reference the folio's pfns,
and use the va to pa mapping info to figure out if its within the specified range.

I don't really understand why the vma is useful.

> +{
> +	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;

Why is this pgoff calculation helpful? Surely it's only useful if the folio
belongs to the same file that the vma is mapping? Otherwise the folio's pgoff
might be referring to a completely different file than the vma's vm_pgoff. So
you will get spurious results.

> +
> +	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;

I don't understand why you don't need to check the pte for a small folio? As
above, if the folio doesn't belong to the file that the vma is mapping the above
checks seem wrong and you can't conclude that the folio is mapped in the range
without looking at the pte?

> +
> +	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);

I think this could wander off the front or back of the pte table into arbitrary
memory if the folio is strddling multiple pte tables.

> +	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)

Doesn't !pte_present() also cover pte_none()? So I think the pte_none() check is
redundant?

Thanks,
Ryan


> +			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,