linux-kernel - Re: [PATCH] mm/huge_memory.c: use helper range_in_vma() in __split_huge_p[u|m]d

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Date:   Mon, 1 Feb 2021 13:27:36 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/huge_memory.c: use helper range_in_vma() in
 __split_huge_p[u|m]d_locked()

On Mon, 1 Feb 2021, Miaohe Lin wrote:

> The helper range_in_vma() is introduced via commit 017b1660df89 ("mm:
> migration: fix migration of huge PMD shared pages"). But we forgot to
> use it in __split_huge_pud_locked() and __split_huge_pmd_locked().
> 
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/huge_memory.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 987cf5e4cf90..33353a4f95fb 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1959,8 +1959,7 @@ static void __split_huge_pud_locked(struct vm_area_struct *vma, pud_t *pud,
>  		unsigned long haddr)
>  {
>  	VM_BUG_ON(haddr & ~HPAGE_PUD_MASK);
> -	VM_BUG_ON_VMA(vma->vm_start > haddr, vma);
> -	VM_BUG_ON_VMA(vma->vm_end < haddr + HPAGE_PUD_SIZE, vma);
> +	VM_BUG_ON_VMA(!range_in_vma(vma, haddr, haddr + HPAGE_PUD_SIZE), vma);
>  	VM_BUG_ON(!pud_trans_huge(*pud) && !pud_devmap(*pud));
>  
>  	count_vm_event(THP_SPLIT_PUD);
> @@ -2039,8 +2038,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  	int i;
>  
>  	VM_BUG_ON(haddr & ~HPAGE_PMD_MASK);
> -	VM_BUG_ON_VMA(vma->vm_start > haddr, vma);
> -	VM_BUG_ON_VMA(vma->vm_end < haddr + HPAGE_PMD_SIZE, vma);
> +	VM_BUG_ON_VMA(!range_in_vma(vma, haddr, haddr + HPAGE_PMD_SIZE), vma);
>  	VM_BUG_ON(!is_pmd_migration_entry(*pmd) && !pmd_trans_huge(*pmd)
>  				&& !pmd_devmap(*pmd));
>  

This actually loses information, right?  Before the patch, we can 
determine which conditional is failing because we know the line number 
that the VM_BUG_ON() is happening on.  After the patch, we don't know 
this.

I don't think that's crucial, but I'm not sure it makes sense to do this 
if the only upside is that we removed one total line of code :)