linux-kernel - Re: [PATCH v2 1/3] mm: add functions folio_in_range() and folio_within

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Message-ID: <241cb5d6-0941-4dbb-9d16-9226d34ebaf8@arm.com>
Date:   Wed, 9 Aug 2023 20:34:21 +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, hughd@...gle.com,
        yosryahmed@...gle.com, david@...hat.com, shy828301@...il.com
Subject: Re: [PATCH v2 1/3] mm: add functions folio_in_range() and
 folio_within_vma()

On 09/08/2023 07:11, 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 | 35 +++++++++++++++++++++++++++++++++++
>  1 file changed, 35 insertions(+)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index 154da4f0d557..5d1b71010fd2 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -585,6 +585,41 @@ 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);
> +
> +static inline bool
> +folio_in_range(struct folio *folio, struct vm_area_struct *vma,
> +		unsigned long start, unsigned long end)

I still think it would be beneficial to have a comment block describing the
requirements and behaviour of the function:

 - folio must have at least 1 page that is mapped in vma
 - the result tells you if the folio lies within the range, but it does not tell
you that all of its pages are actually _mapped_ (e.g. they may not have been
faulted in yet).

 - I think [start, end) is intended intersect with the vma too? (although I'm
pretty sure sure the logic works if it doesn't?)

> +{
> +	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 > end)
> +		return false;
> +
> +	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 (!in_range(pgoff, vma->vm_pgoff, vma_pglen))
> +		return false;
> +
> +	addr = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
> +
> +	return !(addr < start || end - addr < folio_size(folio));
> +}
> +
> +static inline bool
> +folio_within_vma(struct folio *folio, struct vm_area_struct *vma)

why call this *within* but call the folio_in_range() *in*? Feels cleaner to use
the same word for both.

> +{
> +	return folio_in_range(folio, vma, vma->vm_start, vma->vm_end);
> +}
> +
>  /*
>   * mlock_vma_folio() and munlock_vma_folio():
>   * should be called with vma's mmap_lock held for read or write,