linux-kernel - Re: [PATCH 1/3] mm: add functions folio_in_range() and folio_within

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Message-ID: <55c9e3f7-099d-6f57-32da-1f318a9688a0@arm.com>
Date:   Wed, 2 Aug 2023 12:14:14 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     Yin Fengwei <fengwei.yin@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        yuzhao@...gle.com, willy@...radead.org, david@...hat.com,
        shy828301@...il.com, hughd@...gle.com
Subject: Re: [PATCH 1/3] mm: add functions folio_in_range() and
 folio_within_vma()

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@...el.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)

This api seems a bit redundant to me. Wouldn't it be better to remove the vma
parameter and instead fix up the start/end addresses in folio_within_vma()?

> +{
> +	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;
> +
> +	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;
> +	}

I don't think I see anything to ensure you don't wander off the end (or start)
of the pgtable? If the folio is mremapped so that it straddles multiple tables
(or is bigger than a single table?) then I think pte can become invalid? Perhaps
you intended start/end to always be within the same pgtable, but that is not
guarranteed in the case that folio_within_vma() is making the call.

Also I want to check that this function is definitely always called under the
PTL for the table that pte belongs to?

> +
> +	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,