linux-kernel - Re: [PATCH 2/8] mm/memory-failure.c: avoid walking page table when vma

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Date:   Mon, 14 Feb 2022 23:48:26 +0900
From:   Naoya Horiguchi <naoya.horiguchi@...ux.dev>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, naoya.horiguchi@....com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/8] mm/memory-failure.c: avoid walking page table when
 vma_address() return -EFAULT

On Thu, Feb 10, 2022 at 10:17:27PM +0800, Miaohe Lin wrote:
> It's unnecessary to walk the page table when vma_address() return -EFAULT.
> Return early if so to save some cpu cycles.
> 
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>

Acked-by: Naoya Horiguchi <naoya.horiguchi@....com>

Does this patch fix the real problem rather than just saving cpu cycles?
Without this patch, "address == -EFAULT" seems to make pgd_offset() return
invalid pointer and result in some serious result like general protection fault.
If that's the case, this patch might be worth sending to stable.

Thanks,
Naoya Horiguchi

> ---
>  mm/memory-failure.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index b3ff7e99a421..f86819145ea8 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -315,6 +315,8 @@ static unsigned long dev_pagemap_mapping_shift(struct page *page,
>  	pmd_t *pmd;
>  	pte_t *pte;
> 
> +	if (address == -EFAULT)
> +		return 0;
>  	pgd = pgd_offset(vma->vm_mm, address);
>  	if (!pgd_present(*pgd))
>  		return 0;
> ---
> 2.23.0
>