linux-ext4 - Re: [PATCH 2/8] mm: khugepaged: remove redundant check for VM_NO

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Date:   Tue, 1 Mar 2022 17:07:07 +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 2/8] mm: khugepaged: remove redundant check for
 VM_NO_KHUGEPAGED

On 2022/3/1 7:57, Yang Shi wrote:
> The hugepage_vma_check() called by khugepaged_enter_vma_merge() does
> check VM_NO_KHUGEPAGED. Remove the check from caller and move the check
> in hugepage_vma_check() up.
> 
> More checks may be run for VM_NO_KHUGEPAGED vmas, but MADV_HUGEPAGE is
> definitely not a hot path, so cleaner code does outweigh.
> 
> Signed-off-by: Yang Shi <shy828301@...il.com>
> ---
>  mm/khugepaged.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 131492fd1148..82c71c6da9ce 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -366,8 +366,7 @@ int hugepage_madvise(struct vm_area_struct *vma,
>  		 * register it here without waiting a page fault that
>  		 * may not happen any time soon.
>  		 */
> -		if (!(*vm_flags & VM_NO_KHUGEPAGED) &&
> -				khugepaged_enter_vma_merge(vma, *vm_flags))
> +		if (khugepaged_enter_vma_merge(vma, *vm_flags))
>  			return -ENOMEM;
>  		break;
>  	case MADV_NOHUGEPAGE:
> @@ -446,6 +445,9 @@ static bool hugepage_vma_check(struct vm_area_struct *vma,
>  	if (!transhuge_vma_enabled(vma, vm_flags))
>  		return false;
>  
> +	if (vm_flags & VM_NO_KHUGEPAGED)
> +		return false;
> +

This patch does improve the readability. But I have a question.
It seems VM_NO_KHUGEPAGED is not checked in the below if-condition:

	/* Only regular file is valid */
	if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && vma->vm_file &&
	    (vm_flags & VM_EXEC)) {
		struct inode *inode = vma->vm_file->f_inode;

		return !inode_is_open_for_write(inode) &&
			S_ISREG(inode->i_mode);
	}

If we return false due to VM_NO_KHUGEPAGED here, it seems it will affect the
return value of this CONFIG_READ_ONLY_THP_FOR_FS condition check.
Or am I miss something?

Thanks.

>  	if (vma->vm_file && !IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) -
>  				vma->vm_pgoff, HPAGE_PMD_NR))
>  		return false;
> @@ -471,7 +473,8 @@ static bool hugepage_vma_check(struct vm_area_struct *vma,
>  		return false;
>  	if (vma_is_temporary_stack(vma))
>  		return false;
> -	return !(vm_flags & VM_NO_KHUGEPAGED);
> +
> +	return true;
>  }
>  
>  int __khugepaged_enter(struct mm_struct *mm)
>