linux-ext4 - Re: [PATCH 3/8] mm: khugepaged: skip DAX vma

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Date:   Wed, 2 Mar 2022 11:21:07 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Yang Shi <shy828301@...il.com>
CC:     <linux-mm@...ck.org>, <linux-fsdevel@...r.kernel.org>,
        <linux-ext4@...r.kernel.org>, <linux-xfs@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <vbabka@...e.cz>,
        <kirill.shutemov@...ux.intel.com>, <songliubraving@...com>,
        <riel@...riel.com>, <willy@...radead.org>, <ziy@...dia.com>,
        <akpm@...ux-foundation.org>, <tytso@....edu>,
        <adilger.kernel@...ger.ca>, <darrick.wong@...cle.com>
Subject: Re: [PATCH 3/8] mm: khugepaged: skip DAX vma

On 2022/3/1 7:57, 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.

Yep. There is PageLRU check in khugepaged_scan_{file,pmd}.

> 
> So it is not harmful, but it is definitely pointless to run khugepaged
> against DAX vma, so skip it in early check.
> 

This patch looks good to me. Thanks.

Reviewed-by: Miaohe Lin <linmiaohe@...wei.com>

> Signed-off-by: Yang Shi <shy828301@...il.com>
> ---
>  mm/khugepaged.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 82c71c6da9ce..a0e4fa33660e 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -448,6 +448,10 @@ 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;
>