linux-kernel - Re: [v3 PATCH 3/8] mm: khugepaged: skip DAX vma

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Date:   Mon, 9 May 2022 14:46:39 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Yang Shi <shy828301@...il.com>, kirill.shutemov@...ux.intel.com,
        linmiaohe@...wei.com, songliubraving@...com, riel@...riel.com,
        willy@...radead.org, ziy@...dia.com, tytso@....edu,
        akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v3 PATCH 3/8] mm: khugepaged: skip DAX vma

On 4/4/22 22:02, Yang Shi wrote:
> The DAX vma may be seen by khugepaged when the mm has other khugepaged
> suitable vmas.  So khugepaged may try to collapse THP for DAX vma, but
> it will fail due to page sanity check, for example, page is not
> on LRU.
> 
> So it is not harmful, but it is definitely pointless to run khugepaged
> against DAX vma, so skip it in early check.
> 
> Reviewed-by: Miaohe Lin <linmiaohe@...wei.com>
> Acked-by: Song Liu <song@...nel.org>
> Signed-off-by: Yang Shi <shy828301@...il.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  mm/khugepaged.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 7d197d9e3258..964a4d2c942a 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -447,6 +447,10 @@ static bool hugepage_vma_check(struct vm_area_struct *vma,
>  	if (vm_flags & VM_NO_KHUGEPAGED)
>  		return false;
>  
> +	/* Don't run khugepaged against DAX vma */
> +	if (vma_is_dax(vma))
> +		return false;
> +
>  	if (vma->vm_file && !IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) -
>  				vma->vm_pgoff, HPAGE_PMD_NR))
>  		return false;