linux-kernel - Re: [PATCH] fix judgment error in shmem_is

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Date:   Wed, 8 Sep 2021 17:58:44 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Liu Yuntao <liuyuntao10@...wei.com>
Cc:     hughd@...gle.com, akpm@...ux-foundation.org,
        kirill.shutemov@...ux.intel.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, wuxu.wu@...wei.com,
        liusirui@...wei.com, windspectator@...il.com
Subject: Re: [PATCH] fix judgment error in shmem_is_huge()

On Wed, Sep 08, 2021 at 06:26:48PM +0800, Liu Yuntao wrote:
> In the case of SHMEM_HUGE_WITHIN_SIZE, the page index is not rounded
> up correctly. When the page index points to the first page in a huge
> page, round_up() cannot bring it to the end of the huge page, but
> to the end of the previous one.
> 
> an example:
> HPAGE_PMD_NR on my machine is 512(2 MB huge page size).
> After allcoating a 3000 KB buffer, I access it at location 2050 KB.
> In shmem_is_huge(), the corresponding index happens to be 512.
> After rounded up by HPAGE_PMD_NR, it will still be 512 which is
> smaller than i_size, and shmem_is_huge() will return true.
> As a result, my buffer takes an additional huge page, and that
> shouldn't happen when shmem_enabled is set to within_size.
> 
> Fixes: f3f0e1d2150b2b ("khugepaged: add support of collapse for tmpfs/shmem pages")
> Signed-off-by: Liu Yuntao <liuyuntao10@...wei.com>
> ---
>  mm/shmem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 88742953532c..5747572859d1 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -490,7 +490,7 @@ bool shmem_is_huge(struct vm_area_struct *vma,
>  	case SHMEM_HUGE_ALWAYS:
>  		return true;
>  	case SHMEM_HUGE_WITHIN_SIZE:
> -		index = round_up(index, HPAGE_PMD_NR);
> +		index = round_up(index + 1, HPAGE_PMD_NR);
>  		i_size = round_up(i_size_read(inode), PAGE_SIZE);
>  		if (i_size >= HPAGE_PMD_SIZE && (i_size >> PAGE_SHIFT) >= index)

With the change, the condition can be simplified to

		if (i_size >> PAGE_SHIFT >= index)

right?

>  			return true;
> -- 
> 2.23.0
> 

-- 
 Kirill A. Shutemov